This is an advanced level course. After completing this course, mental health professionals will be able to:
This course is divided into three sections: (1) History of the concept of EF, (2) problems and commonalities across the diversity of EF conceptualizations, and (3) problems with the tests used in the assessment of EF.
This course is adapted from chapters in Dr. Barkley's latest book, Barkley, R. A. (2011). Executive Functioning and Self-Regulation: Integration, Extended Phenotype, and Clinical Implications. New York: Guilford Press. It is also based in part on content from his latest manual, Barkley, R. A. (2011). The Barkley Deficits in Executive Functioning Scale. New York: Guilford Press.
The history of the concept of executive functioning (EF) appears to begin more than 140 years ago in the 1870s in the initial efforts by scientists to understand the functions of the frontal lobes generally and the prefrontal cortex specifically (Luria, 1966). Indeed, its history predates the use of the very term EF by nearly 100 years given that the term EF itself arose out of earlier efforts to understand the neuropsychological functions mediated by the frontal, and especially the prefrontal or premotor, regions of the brain. Thus, there is an inherent conflating of the term EF with the functions of the prefrontal cortex (PFC) and vice versa that exists because of this history. Born out of the initial efforts to understand those PFC functions, the concept of EF was at first defined by default as what the prefrontal lobes do (Stuss & Benson, 1986). Particular attention was paid to the PFC rather than the more obvious, pedestrian, non-executive functions of the primary, secondary, and tertiary motor programming zones of the larger frontal lobes to be found adjacent and more anterior to the sensory-motor strip.
Over time, this conflation has led to a circularity of reasoning in that the functions of the PFC are said to be EF while EF is then defined back to the functions of the PFC. It has also led to a slippage in the discourse on EF between two separate levels of analysis, one being the neuropsychological level involving thought (cognition), emotion, and verbal or motor action (behavior) and the other being the neuro-anatomical level involving the localization of those functions of the former level to specific regions of the brain and their physiological activity (Denckla, 1996). As Denckla (1996) and others have rightly noted, EF is not exclusively a function of the PFC given that the PFC has various networks of connections to other cortical and subcortical zones as well as the basal ganglia, amygdala and limbic system, and cerebellum (Fuster, 1997; Luria, 1966; Stuss & Benson, 1986). Moreover, the PFC may well engage in certain neuropsychological functions that would not be considered to fall under the umbrella of EF, such as simple or automatic sensory-motor activities, speech, and olfactory identification.
Luria (1966) gives a fine account of the beginning history of the study of the functions of the frontal lobes on which I rely here to convey this historical period. From him we learn that Hughlings Jackson is said to have viewed the PFC as "the highest motor centres" being "the most complex and least organized centres . . ." (Luria, 1966, p. 221). According to Luria (1966), Bianchi (1895) voiced similar views independently of Jackson arguing that the frontal lobes contained the most complex forms of reflex activity organized hierarchically into a series of levels that "bring about the widest coordination of sensory and motor elements, utilize the product of the sensory zones to create mental syntheses, and play the same role in relation to the sensorimotor (or kinesthetic) zones that the latter play in relation to the subcortical nuclei." (p. 221, Luria, 1966). This was the integrative function first attributed to the frontal lobes. It was based largely on animal ablation studies that resulted in a disorganized, fragmented, and unsubordinated character to the behavior of animals that had their PFC extirpated. This caused the animal to "cease to adapt itself to new conditions." (p. 221, Luria, 1966). At this same time, others apparently attributed an inhibitory function over the lower divisions of the brain to the frontal lobes based on evidence that frontal ablations often led to a release of lower automatisms.
Noteworthy to Luria was the work of Bekhterev (1905-1907) who, in his text, Fundamentals of Brain Function, observed that damaging the frontal lobes resulted in a disintegration of goal-directed behavior, which he saw as the principle function of the PFC. This statement is historically noteworthy, as it appears to be the origin of modern definitions of EF as being those processes that support goal-directed behavior (Welsh & Pennington, 1988). Damage to the PFC specifically led to a loss of the individual's evaluation of their actions relative to their initial goals. It also led to difficulties correctly evaluating external impressions relative to those goals. Hence, there was a decline in the purposive and deliberate choice of movements in accordance with such evaluations and goals (p. 222). During the early 1900s, Pavlov also was said to have noted the purposive nature of the PFC and the loss of "the integration of goal-directed movement" as a consequence of damage to this region (see p. 222, Luria, 1966). By the 1960s, Luria notes, "one of the essential results of destruction of the frontal lobes in animals is a disturbance of the preliminary . . . syntheses underlying the regulation of complex forms of motor operations and the evaluation of the effect of their own actions, without which goal-directed, selective behavior is impossible." (p. 224).
Additional research by others at this time would also show that animals deprived of their PFC were "easily distracted by extraneous stimuli" (p. 224) including extraneous thoughts or patterns of irrelevant mental associations (p. 286), were unable to develop or sustain a readiness for action or intentional quality to their actions, and were hyperactive as a consequence of the poor inhibition of the lower, more automatic forms of behavior. Hence, the integration and execution of goal directed behavior, the inhibition of more automatic actions and reactions to extraneous stimuli (distractibility), the production of delayed reactions, the evaluation of one's goal-directed actions relative to the external environment – especially in novel circumstances – and the overall intentionality or purposive quality of behavior were all functions attributable to the PFC during the first 30-80 years of the scientific study of its functions. This makes it easy to understand just how such functions became bundled into the later conceptualization of EF when that specific term eventually arose to describe the neuropsychology of the PFC.
Luria (1966) noted that published reports of humans suffering damage to various regions of the PFC would likewise have symptoms resembling those observed in animals whose PFC had been experimentally extirpated. Among the first and most famous of such cases was that of Phineas Gage, the railroad foreman who suffered a penetrating head wound that destroyed a large portion of his PFC. This led to drastic alterations in his behavior, personality, and social conduct (Harlow, 1848, 1868). Like Gage, patients with PFC damage studied in these early years demonstrated a lack of initiative or drive, a curtailing of their circle of interests, profound disturbances of goal-directed behavior, a loss of abstract or categorical behavior, and emotional changes, such as a proneness to irritation, emotional instability, and indifference toward their surroundings, often superimposed on depression. Impulsive actions, trivial jokes, and even euphoria were noted to arise from lesions that involved the more basal aspects of the PFC. Just as likely was also an adverse impact on moral conduct, independence and self-reliance, financial-economic self-support, effective occupational performance, and socially cooperative activities that all require the capacity for evaluating the longer-term consequences of one's actions as noted in the initial report on Gage (Harlow, 1848).
Luria goes on to state that "besides the disturbance of initiative and the other aforementioned behavioral disturbances, almost all patients with a lesion of the frontal lobes have a marked loss of their 'critical faculty,' i.e., a disturbance of their ability to correctly evaluate their own behavior and the adequacy of their actions." (p. 227, Luria, 1966). Impaired here was a faculty of relational comparisons between the individual's goals, their current actions relative to those goals, their actions relative to others while in pursuit of those goals, and the environmental feedback as to the effectiveness of those actions vis a vis the goal. This is similar to the observations of Freeman and Watts (1941) that the frontal lobes are concerned with foresight and the relation of the self (current) to the self (future).
PFC injured patients also demonstrated a marked impairment in voluntary movement and activity. Such movements are unique to humans. They are a response to either verbal instruction from others or the formation of an intention of their own translated into a self-instruction. Today, these would be considered forms of rule-governed behavior (Hayes, Gifford, & Ruckstuhl, 1996) or what Luria called the regulating functions of speech (Luria, 1966, p. 250).
Damage to the PFC and this array of impairments were typically accompanied by a release of more automatic forms of behavior. For instance, this might occur in the inappropriate utilization of an object for its intended purposes in the wrong context as described by Lhermitte, Pillon, and Serdaru (1986). It would also be manifest in the perseveration of actions despite a change in the context that should have led to a termination of those actions and in responding impulsively to irrelevant events (Luria, 1966). The totality of this pattern of deficits came to be known as a frontal lobe syndrome. Luria (1966) distinguished among at least three such syndromes each related to the particular frontal region that had been destroyed (dorsolateral, ventro-medial, orbital).
The first use of the term "executive" in reference to PFC functions has been attributed by others (Dimond, 1980; Fuster, 1997) to Karl Pribram (1973, 1976). He stated that:
". . . the frontal cortex is critically involved in implementing executive programmes where these are necessary to maintain brain organization in the face of insufficient redundancy in input processing and in the outcomes of behavior."
Others (Wolf & Wallerstein, 2001) have credited Butterfield and Belmont (1977) for the term but their use of it post-dates that of Pribram and so historical precedence goes to Pribram in most accounts of this term. It is here, then, some 100 years after the initiation of scientific interest in the functions of the PFC that its over-arching "executive" nature is declared. The PFC is the brain's executive. Just what features make a function executive in nature or not were not clearly specified by Pribram.
In time, the frontal lobe syndrome noted by Luria (1966) would evolve into an executive disorder (Fuster, 1997) or a "dysexecutive syndrome" (Baddeley, 1986; Wilson, Alderman, Burgess, Emslie, & Evans, 1996). Over the next 40 years, the frequency of use of the term EF would explode to become one of the most common terms to appear in neuropsychological journals. It would also be synonymous with the functions of the PFC. This conflating of distinct levels of analysis in science – the neurological with the psychological – would occur despite, as Denckla (1996) noted, this linkage being completely unnecessary to the theorizing or research occurring at each of these two different levels. The definition of EF would also increase in diversity to a point where 33 different concepts would be associated with the term by leading experts in neuropsychology (Eslinger, 1996), eventually reaching a point where EF could mean pretty much what the investigator wished it to be for the sake of their own investigative aims, as noted by Wolf and Wallerstein (2001).
A brief examination of these various definitions is worthwhile not only to illustrate the diversity of constructs placed under the umbrella (more like the tent) of EF, but also to seek out any commonalities that may prove useful in laying a foundation for further theory development.
This section represents an attempt merely to provide a sampling of the wide array of definitions that have appeared in the literature on EF. It is not intended to be comprehensive or exhaustive but it is probably representative.
A commonly cited early conceptualization of EF was that specified by Stuss and Benson (1986) in their book on the frontal lobes:
"Executive control functions, called into action in nonroutine or novel situations, provide conscious direction to the functional systems for efficient processing of information."(p. 244) The executive function represents many of the important activities that are almost universally attributed to the frontal lobes that become active in nonroutine, novel situations that require new solutions. These behavioral characteristics have been described by many authors and include at least the following: anticipation, goal selection, preplanning (means-end establishment), monitoring, and use of feedback (if-then statements)." (p. 244)
In this definition, EF refers to four components (anticipation, goal selection, pre-planning, and monitoring). In their diagram of PFC functions, Stuss and Benson (1986) place these EF components above two other frontal modules which they are said to govern: Drive (drive, motivation, and will) and Sequencing (sequence, set, and integration). In this model, drive and sequencing are not EFs. Drive, motivation, and will comprise the first (Drive) of the two modules governed by the EF control system (p. 243). In it, drive refers to basic appetitive states that are basic energizing forces. Motivation is conceived as being more mental/intellectual control of drive states. And "will" is undefined but is implied to be an even higher state that governs motivation, most likely representing consciously conceived wants or desires.
The second of these modules (Sequencing) is said to be involved in organizing and maintaining bits of information into meaningful sequences, such as in the temporal integration and sequencing of behavior. Stuss and Benson cite Fuster's work in support of the existence of this module (See Fuster, 1997). Fuster argued that three subordinate functions are needed to organize and integrate behavior across time, these being anticipation (the prospective function), provisional memory (working memory), and control of interference (an inhibitory function). To these, Stuss and Benson added the synthetic capacity to form sets of related information that allows the production of new, more complex information from available sequences of data. With this, also goes the capacity for the integration of a number of related and unrelated sets of information into novel knowledge and hence novel action (see pp. 241-242).
These two modules (Drive, Sequencing) govern the nonexecutive posterior/basal functional systems, such as attention, alertness, visual-spatial, autonomic emotional, memory, sensory/perception, language, motor, and cognition. Interestingly, perched above the EF level and atop all of these components is Self-Awareness, believed to be the highest attribute of the frontal lobes (see Figure 17-4 in Stuss & Benson, 1986). It is viewed as separable from EF and hierarchically placed above it (p. 246-247). Noteworthy from this perspective is that self-awareness is implied, if not declared, to be the central executive that determines the activities of the lower level functions, including the EF level.
One of the most popular definitions appearing in the literature on EF, particularly in research on normal children and people having ADHD (Hinshaw, Carte, Fan, Jassy, & Owen, 2007; Martel, Nikolas, & Nigg, 2007; Rhodes, Coghill, & Matthews, 2005; Wilding, 2005; Willcutt et al., 2005), is that of Welsh and Pennington (1988):
"Executive function is defined as the ability to maintain an appropriate problem-solving set for attainment of a future goal." They include the components of: "a) an intention to inhibit a response or to defer it to a later more appropriate time; b) a strategic plan of action sequences; and c) a mental representation of the task, including the relevant stimulus information encoded in memory and the desired future goal-state." (pp. 201-202)
While many of the authors citing this definition attribute it to Welsh and Pennington (1988), the latter authors credit it to Luria (1966) and even earlier, Bianchi (1922). And we know from above that Luria (1966) actually credited the idea to the earlier writings of Bekhterev (1905-1907). Yet Luria, Bianchi, and Bekhterev were not defining EF – they were simply describing what they felt were important functions of the PFC. Here again we see a conflating of EF = PFC functions and an unnecessary circularity of reasoning.
The Welsh and Pennington (1988) definition appears to contain four elements of EF, these being intentionality (goal-directedness), inhibition, planning, and working memory (mentally representing information). Later, Roberts and Pennington (1996; Pennington, 1997) would reduce their model to just two components (working memory and inhibition) that interact to resolve conflicts or competing action alternatives. Yet they acknowledged that EF was used by others as an umbrella term that "refers to a collection of related but somewhat distinct abilities such as planning, set maintenance, impulse control, working memory, and attentional control" (p. 105). They noted that the PFC also serves "a coordinative function, integrating component cognitive-perceptual processes across time and space." (p. 105). They state that these may be reduced to a smaller set of core PFC processes, those being "the ability to maintain and manipulate short-term information needed for generating upcoming action (working memory) and the ability to inhibit inappropriate action."(p. 105).
Pennington, Bennetto, McAleer, and Roberts (1996) later qualified the Welsh and Pennington definition further by saying that any definition of EF "is provisional and general." (p. 327). They acknowledged that EF is often used as an umbrella term for the functions of the PFC. Yet they further noted that some EFs may involve non-PFC regions while some PFC functions are not considered to be executive in nature. No guidance was offered as to just what essential feature would make a function executive or not.
A quite different approach to defining EF was taken by Lezak (1995) in her comprehensive reviews of neuropsychological assessment. To her,
"The executive functions consist of those capacities that enable a person to engage successfully in independent, purposive, self-serving behavior. They differ from cognitive functions in a number of ways. Questions about executive functions ask how and whether a person goes about doing something (e.g., Will you do it and if so, how?); questions about cognitive functions are generally phrased in terms of what or how much (e.g., How much do you know? What can you do?)." (p. 42; Lezak, 1995)
Refreshing here is the emphasis on the social importance of EF – the notion that EF is not just goal directed or purposive in quality, as others have also emphasized, but that it is essential for socially independent behavior and that such behavior is for one's self-interests. It is a point to which we will return later. Lezak goes on to say that:
"The executive functions can be conceptualized as having four components: (1) volition; (2) planning; (3) purposive action; and (4) effective performance. Each involves a distinctive set of activity-related behaviors. All are necessary for appropriate, socially responsible, and effectively self-serving adult conduct. Moreover, it is rare to find a patient with impaired capacity for self-direction and self-regulation who has defects in just one of these aspects of executive functioning. Rather, defective executive behavior typically involves a cluster of deficiencies of which one or two may be especially prominent." (p. 650, Lezak, 1995).
The term "volition" here refers to a conscious choice of actions that are intentional or purposive in nature. Those actions are being guided by a conscious formulation of both a current and future state of the individual and the goal being pursued to satisfy that desired future state. In Lezak's definition, we see a willful intentionality to EF-guided behavior – the individual as an autonomous, freely choosing agent. There is a human dignity in this view that is not seen in other, more "cognitive" views of EF that appear to treat the individual and their EF system as a passive information processor. This definition further declares that there is a conscious generation of those motivational/emotional states that will be needed to initiate and sustain the future-directed action. Hence, there is a strong motivational element to EF in Lezak's view and an important role of emotion in EF – not just a cold cognitive element. Stuss and Benson (1986) and Fuster (1997) also noted this essential motivational aspect to EF. Lezak further notes that effective performance requires a capacity for self-awareness and hence self-monitoring of that ongoing performance against the goal striving to be attained. That self-awareness includes not just awareness of one's physical status but of the environmental and situational context in which one exists, and the social nature of most such contexts (warranting a role for self-consciousness of others).
Planning, in Lezak's (1995) view, was "the identification and organization of the steps and elements (e.g., skills, material, other persons) needed to carry out an intention or achieve a goal . . ." (p. 654). This involves capacities for foresight (i.e. look ahead); objectively evaluating one's self and the context (the abstract attitude); conceiving of alternatives; weighing them, and making choices among them; considering both the sequential and hierarchical structure that was needed to carry out the plan; impulse control; sustained attention (persistence); and memory.
Important for later consideration is that Lezak (1995) emphasized that many of these capacities involved in planning, specifically, and EF more generally (such as volition, choice, intentionality, self-motivation, and self-awareness), would be difficult to examine by testing alone. Information about them was more likely to be derived from the person's history, the clinical observations of the individual across the period of examination, the reports of others who know the person well, and the archival records available for making an indirect ascertainment of deficits.
To Martha Denckla, "EF has become a useful shorthand phrase for a set of domain-general control processes . . . ." (p. 263; Denckla, 1996) She then notes what these processes are likely to involve, such as inhibition and delay of responding, anticipatory set, preparedness to act, freedom from interference, and the ability to sequence behavioral outputs. (p. 265) as well as planning (p. 265). She attributes "distinctive future tense aspects of EF constructs: 'attention to the future,' 'prospective memory,' ('remembering to remember'), or 'memory for the future' are some of the catchphrases employed." (p. 266). Denckla critically notes that EF begins with the unquestioned premise that it is whatever the frontal lobes do. The more functions we learn the frontal lobes do, she states, the more will be added to EF and vice versa. She rightly notes that the linkage of the two levels is a hypothesis, not a given. She also acknowledged a paper by Heilman (1994; cited in Denckla, 1996) presented at a scientific meeting in which he referred to EF as the "how" and "when" of intentionality, fractionated into initiate, sustain, inhibit, shift.
Many others both before and after Denckla (1996) and Lezak (1995) would emphasize the "umbrella" or "family" nature of the concept of EF. This is to say that EF includes a diverse set of cognitive processes within it as well as an overall regulatory, purposive, and goal directed nature. These various definitions, in chronological order, are shown in the boxed Table following this section. Across these various definitions one finds general agreement that EF consists of multiple cognitive functions or abilities (an umbrella, set, family, or multi-dimensional construct), that they might be unified by their being used for the over-arching purpose of goal-directed behavior, and that they are most needed in novel situations, which is also to say when problem-solving may be most needed. After all, a problem by its very definition is a novel situation for which one has no immediately effective response. But this may derive mainly from the fact that many authors, either knowingly or unknowingly, are basing their definitions on the earlier ones noted above by Welsh and Pennington (1988), or Pennington and Roberts (1996)(who based their view on that of Luria who based his on the view of Bekhterev). There is also the suggestion that there is not just diversity, but also some unity to EF that binds the diverse functions into a single umbrella or meta-construct, that over-arching function being apparently goal-directed behavior and associated problem solving. Some have suggested that this shared or central executive may be the working memory or executive attention system (McCabe, Roediger, McDaniel, Balota, & Hambrick, 2010). There may also be some tacit agreement that the EFs are the highest psychological functions of humans and that EF has something to do with self-regulation and volitional conduct, although this is often left out of explicit statements concerning cognitive models of EF (McCabe et al., 2010; Welsh & Pennington, 1988).
Some authors simply skip defining EF entirely (Biederman, Petty, Fried, Black, Faneuil, Doyle, Seidman, & Faraone, 2008; Gilotty, Kenworthy, Sirian, Black, & Wagner, 2002; Papadopoulos, T. C., Panayiotou, G., Spanoudis, G., & Natsopoulos, 2005). They proceed instead to listing one or a few constructs believed to represent EF, such as response inhibition (Hale, Reddy, Decker, Thompson, Henzel et al., 2009), working memory, set shifting, and planning (Castellanos et al., 2006;Willcutt et al., 2005). These constructs are sometimes referred to as "cold" cognitive EF constructs in contrast to the constructs of emotional and motivational self-regulation termed "hot" EFs (Castellanos et al., 2006; Nigg & Casey, 2005; Nigg, Willcutt, Doyle, & Sonuga-Barke, 2005). Such approaches to studying EF are not of much assistance to advancing our understanding of EF nor do they necessarily reveal the relationship of EF to the disorders under study. After all, if EF is not to be defined, then any subsequent relationships found in the study between the test battery and the disorder cannot be easily interpreted.
Largely eschewing an attempt to operationally define the concept of EF, many authors have pursued a more empirical, atheoretical, statistical approach to understanding EF. This has chiefly been exemplified by attempts to factor analyze various batteries of putative EF tests to discover their underlying dimensions. There have been numerous such attempts with most identifying several distinct EF dimensions or factors. For instance, Miyake and colleagues (Miyake, Friedman, Emerson, Witzki, Howerter, & Wager, 2000) used confirmatory factor analysis to determine if the three most commonly proposed EFs did, in fact, form distinctive functions – these being set shifting, information updating, and inhibition. Using three tasks commonly used to assess these constructs and college students as participants, Miyake et al. found evidence for such a fractionating of EF. However, Shute and Huertas (1990) also gave a battery of seven EF tasks to college students and identified four factors – flexibility/perseveration (or set shifting), perceptual-motor speed (probably not an EF), verbal working memory, and time estimation. These factors mainly reflected the tests in the battery. In Levin and colleagues' study of brain-injured children (Levin, Fletcher, Kufera, Harward, Lilly, et al., 1996), five factors were identified from their battery of six EF tasks. Grodzinsky and Diamond (1992) gave a set of 10 EF tests to their participants and found at least seven separate factors, most of which reflected the tests. Mariani and Barkley (1997) also used an extensive battery of EF and non-EF tests and identified at least four factors.
Anderson (2002) described the results of several factor analyses of EF test batteries and the common factors they identified. These were planning, impulse control, and concept reasoning. A response speed factor was also found, but this would be considered non-executive. He then goes on to develop his own four factor model of EF (1) attentional control (includes inhibition); (2) information processing (fluency, efficiency, and speed of output); (3) cognitive flexibility (shift response sets, learn from mistakes, devise alternative strategies, divide attention, process multiple sources of information concurrently; and (4) goal setting (develop new initiatives and concepts, plan actions in advance, approach tasks in an efficient and strategic manner).
However, all of these findings could just as easily reflect method variance in that, as a general rule, the more tests included in the EF battery, the more factors the study seems to identify – these factors primarily comprise the different tests. It is therefore not clear that EF is as fractionated a construct as such studies suggest; nothing requires that it be conceptualized as such. What these studies more likely illustrate is that a diversity of putative tests of EF, when factor analyzed, result in a diversity of dimensions – as diverse as the tests included in the battery. Also troubling in this empirical (statistical) approach is that typically the correlations among these various factors are relatively and disturbingly low, suggesting only 10-20% of shared variance among them – a finding not encouraging of a central construct or general EF factor (Lehto, 1996). However, as Miyake and colleagues noted (2000), this limited relationship among the factors could just as easily reflect the diversity of non-EF abilities that are also being sampled in the EF test batteries. These non-EF abilities would obscure the commonality that might exist among them that would represent the higher order central executive faculty lying latent across these diverse tasks.
Another major problem with such an empirical approach to identifying the components of EF is that, in the absence of a consensus or adequate definition of EF, it is difficult to evaluate whether or not each of the various tests being included in these batteries actually assess EF (a problem to be discussed further in Section 2). As reflected in these and many other factor analytic studies of EF batteries, the number of tasks believed to evaluate EF is substantial, seemingly limited only by the number of investigators studying the construct as Wolf and Wallerstein (2001) duly noted. That is to be expected when researchers have no compelling theory or no consensus definition available to them in the literature and so are left to their own devices and opinions to define the term or its components and then select the tasks to be used to evaluate it. The end results is a veritable Heinz-57 test battery of EF that, when factor analyzed, coughs up nearly as many components or factors as there are tests in the battery. The circularity of reasoning here becomes obvious when, on the one hand we are told that EF is whatever these tests are measuring and then turn around on the other hand and claim that the tests and their factorial dimensions are telling us the nature of EF.
To make this situation even more confusing, tests believed to evaluate EF by some researchers have also been claimed to be measuring different components of attention by others. Most notable has been the extensive program of research by Mirsky (1996) to discern the different forms of attention. The battery he used is comprised of tasks that others would have considered EF, such as the Wisconsin Card Sort Test, a continuous performance test, verbal working memory subtests from the Wechsler intelligence test, and a digit cancellation test. Like the statistical approach to discerning EF, he identified nearly as many factors has he had tests. Four separate factors emerged, some even bearing names used for those found in factor analyses of EF batteries, such as "Inhibit" and "Shift." Likewise, in study of normal adults focusing on the components of attention, Robertson and colleagues (Robertson, Ward, Ridgeway, & Nimmo-Smith, 1996) discovered at least four factors in their battery that they labeled as Visual Selective Attention/Speed, Attention Switching, Sustained Attention, and Auditory/Verbal Working Memory. Again, several of these would also be considered by others to reflect EF (set shifting, working memory, and persistence toward goals). Therefore, this empirical (factor analytic) approach to discovering the nature of EF has not been as fruitful to date as some researchers have proclaimed (Anderson, 2002; Miyake et al., 2000). If a poorly defined umbrella construct of EF is used to select a diversity of putative EF tests, then a diversity of components is often the result from any factor analysis. This does not get one out of the trap that the construct itself was originally so poorly or so broadly defined (operationalized) as to include no standard by which to select or eliminate potential tests from the battery. Hackneyed as it may seem, the phrase "garbage in, garbage out" for factor analyses is still nonetheless true. Absent a precise structural hypothesis (operational definition of the components of the construct), little sense can be made of the results.
Some models of relevance to this historical discourse were not originally designed to address the meaning of EF per se but to explain some of the functions of the PFC. Given the widespread view that EF is essentially the functions of the PFC, however, they are at least worth considering in attempts to understand the nature of EF.
A widely cited theory of PFC functioning that similarly deals with goal-directed behavior is Fuster's model of cross-temporal synthesis or integration (1997). Cross-temporal synthesis is based on three PFC components: (1) working memory, which is a temporally retrospective function; (b) anticipatory set (planning), which is a temporally prospective function; and (c) interference control (a form of attention that involves resistance to distraction), which inhibits the disruption of goal-directed behavior by events or behavior that are irrelevant to or incompatible with the goal. Fuster argues that the over-arching purpose of these three EF components is "the cross-temporal organization of behavior." (1997, p. 157) This is achieved by a temporal synthesis or integration that represents the "formation of temporal structures of behavior with a unifying purpose or goal – in other words, the structuring of goal-directed behavior." (p. 158) Like Stuss and Benson (1986), Fuster includes in his concept of EF the self-regulation of motivational, emotional, and other drive states in the service of goal-directed behavior.
Central to this model of PFC/EF is the need to appreciate that goal-directed actions often involve significant delays among events (E), responses (R), and goals and their attendant consequences (G/C). Such lengthy delays require an internal means of temporal structuring or binding together the components of this contingency. This, Fuster argues, is done by mentally representing the E-R-G/C arrangement. Goal-directed behavior is achieved through the guidance of behavior by these internal representations and those representations arise from his three components. To Fuster, EF ". . . is closely related, if not identical, to the function of temporal synthesis of action, which rests on the same subordinate functions. Temporal synthesis, however, does not need a central executive." (p. 165) Fuster does not put a ghost in the machine or homunculus in the mind. All along, it is the individual organism that is selecting what goals it will pursue within the constraints of its time horizon or capacity for retrospective and prospective functions.
While Fuster acknowledges that the PFC-injured patient displays an inordinate degree of concreteness in their daily behavior, he believes that this is largely – if not solely – a temporal concreteness. "The patient suffers from an overall constriction of the scope and complexity of behavior and of the thinking behind it." (p. 165). Behavioral patterns that are not well established are anchored in the present, devoid of temporal perspective, for the past as well as the future, and have an air of temporal immediacy dominated by immediate needs and stimuli or the here and now. In this model, once a desire, want, or goal comes into mind, the temporal integration or synthesis activities of the frontal lobes serve to construct the cross-temporal chains of behavior necessary to its attainment and, if thwarted, the problem-solving mental manipulations that may be needed to surmount the obstacle. Deficits in any of the three can result in deficient temporal integration of behavior, or EF, and thus lead to different forms or origins of EF deficits. EF (temporal synthesis) depends on all three of Fuster's components and hence deficits in any component give rise to a distinct disorder of EF.
A less complex theory of the functions of the PFC is that of goal-neglect by Duncan (1986). It essentially argues that human behavior is organized around a list of goals and sub-goals against which the individual is comparing their ongoing behavior to maintain behavior directed at these goals. Frontal lobe damage results in an inability to retain these goals in mind and thus greater disorganization of behavior.
Most of the definitions or descriptions of the functions of the PFC given above arose out of clinical observations of patients with frontal lobe injuries or ablation studies of animals. An alternative perspective to EF developed in the 1990s out of information processing theory. Typical of this was the work of Borkowski and Burke (1996) and other authors whose work they summarized in this field. Borkowski and Burke (1996) described EF as a set of three components that are directed at problem solving: Task analysis (essentially defining the problem), strategy selection and revision, and strategy monitoring. Those authors also cited a different information-processing model developed by Butterfield and Albertson (1995) that views executive functioning as one of three components of cognition: cognition, metacognition, and executive functioning.
"Cognition is all the knowledge and strategies that exist in long-term memory; this reservoir of information is critical to effective problem solving. The metacognitive level is aware of this lower level and contains models of the various cognitive processes as well as an understanding of how knowledge and strategies interconnect. Executive functioning coordinates these two levels of cognition by monitoring and controlling the use of the knowledge and strategies in concordance with the metacognitive level." (p. 241, Borkowski & Burke, 1996)
Another model from cognitive psychology acknowledged by Borkowski and Burke (1996) in their review was that of Bransford and Stein (1993) and their model of the IDEAL problem-solver that included EF in that model. IDEAL is the acronym for the components of the model which are: (1) identify an important problem to be solved, (2) define the subgoals involved in solving the problem, (3) explore the possible approaches to the problem (select a potential set of strategies), (4) anticipate potential outcomes before acting on the best initial approach; and (5) look back and learn from the entire problem-solving experience. If problem-solving is to be a component of EF, then this usefully makes explicit what steps are being used in that component. The steps here overlap with the Borkowski and Burke model and that of Butterfield and Albertson above. These steps are also highly similar to Scholnick and Friedman's (1993) model of planning. Hence, planning and problem solving may be synonymous terms although planning seems to also include a longer-term future consideration than might problem solving which can be applied to much more near-term problems. Still, both models include a component of problem selection (choosing an important problem implies a futuristic feature perhaps) and certainly anticipating outcomes that include some time horizon over which problems/goals and actions are to be considered.
Borkowski and Burke (1996) admit that self-regulation, planning, and EF overlap but argue that they have some distinctions. Planning, they claim, necessitates decision-making, regulation, and action. They view self-regulation as a component of planning but limited to the strategies necessary to achieve desired goal states. EF like self-regulation is a component of planning because planning has more generality in its application – whereas the former may be less so. EF in their view only involves task analysis and related steps (strategy selection/revision, self-monitoring). These may be distinctions without a difference, or variations on a common theme of goal-directed action. Borkowski and Burke (1996) surprisingly do not include inhibition in their EF model yet admit that deficiencies in it would spill over into their components of EF and be detrimental to them.
A quite different view of EF was proposed at this same time by Hayes, Gifford, and Ruckstuhl (1996) using a more behavioral analytic model and particularly the concept of rule-governed behavior (see Hayes, 1989). Their analysis of the terms often believed to comprise EF as well as many of the tests used to assess EF led them to conclude that EF is a special subset of rule-governed (verbally regulated) behavior. Rule-governed behavior is behavior that is being initiated and guided by verbalizations, whether self-directed or provided by others. We saw elements of the importance of this type of PFC function in the work of Luria (1966) above.
Hayes et al. argued that EF tasks place people in situations where previously learned sources of behavioral regulation come into conflict with rules laid down by the task and examiner. Those task-specific rules are competing with behavior that is otherwise automatic and well-practiced. Thus, the typical automatic flow of behavior must be interrupted and delayed long enough for the person to discover a new rule or select among previously learned rules that may apply in this situation. Yet interrupting a well-practiced behavior itself often requires that a rule be selected and followed that is initiating the delay in responding. And so, in EF tasks an individual often has to implement a rule to inhibit their usual ongoing responding, even if it is just asking them a question about the task. They must then either select from among a set of relevant previously learned rules or generate a new one. The latter is a verbal means by which we problem solve, as in the five steps to problem solving discussed above, which are second order rules used to discover first order rules.
Discovering or selecting the rule is only part of the process, for the individual must now follow it, adhere to it, or "track" the rule. Pliance may occur, where immediate and artificial consequences are applied to motivate the rule following, or tracking may occur where the natural consequences have now taken over to sustain the behavior. Sometimes augmenting is also needed where verbal self-reinforcement (or statements by others) is also used for motivation, as in phrases like "good boy," or self-statements of being right or being good in following the rule. This view of EF appeared to gain little favor among traditional neuropsychologists who seemed more comfortable using a cognitive neuropsychological or information processing view of EF than one derived from behavior analysis. Yet features of it would be incorporated into my own model of EF (Barkley, 1997a, 1997b) through Vygotsky's model of the internalization of self-directed speech.
In 1994, I sketched the broad outlines of a proto-theory of EF at this level and applied it specifically to attention deficit hyperactivity disorder (ADHD), a relatively chronic developmental disorder of inattention, impulsivity, and hyperactivity known to be associated with various deficits in EF (Frazier et al., 2004; Hervey et al.; 2004; Willcutt et al., 2006). This "hybrid" theory was not so much a theory of ADHD but a theory about the nature of EF. That model of EF was then used to show how a particular disorder, in this case ADHD, could be characterized by its adverse impact on normal EF development. That theory was subsequently further developed (Barkley, 1997a) and eventually published as a book (1997b).
Unique to this theory is that it specifies that each EF is a type of self-regulation – a form of self-directed action. And each arises via the same developmental sequence and process – that sequence is best illustrated by the self-direction and internalization of speech described by Vygotsky (1962, 1978, 1987)(see also Diaz & Berk, 1992). Each EF is a form of human behavior (action) that was initially outer-directed – that is, a behavior or set of responses to the environment, usually the social environment. It eventually becomes self-directed during child development as a means by which the individual attempts to alter their subsequent behavior from what it would have otherwise been. This self-directed action is engaged in so as to attempt to alter the likelihood of a delayed (future) consequence. It serves as both a means by which the individual manages or modifies their ongoing behavior and a means to mediate or bridge the delay to the consequence itself. Talking to yourself, counting to yourself, or visualizing task relevant images to yourself are all means young children use to attempt to successfully mediate delays to later consequences and are therefore more likely to obtain those consequences (Mischel, 1983; Mischel, Shoda, & Rodriguez, 1989). In short, each EF is a type of self-regulation. Self-regulation can be defined as any self-directed action that serves to modify a subsequent behavior so as to alter a later consequence. Using the internalization of self-speech as an exemplar, this theory argues that the all EFs arise from and follow a very similar process (self-directed action) and follow a similar developmental sequence (outer directed action becomes self-directed yet observable which then progresses to becoming private and largely unobservable [cognitive or mental in form]). A brief summation of the major components of this theory is provided here. Readers wishing a detailed presentation of its origins, structure, and research support are directed to earlier texts (Barkley, 1997b/2001, 2006).
Six such self-regulatory actions are elaborated in this theory, each of which is a conscious, voluntary, effortful action. The first five are from the model as originally proposed but the sixth has been added of necessity based upon prior reviews of and subsequent research into the nature of EF.
1. Self-Restraint (executive inhibition) – This represents self-stopping or the ability to interrupt the flow of ongoing behavior that serves initially to decouple a stimulus or event from the subsequent response to be prepared for it. It involves (a) the capacity to suppress or otherwise disrupt or prevent the execution of a prepotent or dominant response to an event (the response that has been previously associated with reinforcement or has the highest likelihood of being performed under ordinary circumstances); (b) the capacity to interrupt an ongoing sequence of behavior toward a goal if it is proving to be ineffective; and (c) the capacity to protect the self-directed actions that will subsequently occur and the goal-directed actions they are guiding from interference by external and internal goal-irrelevant events (creating a freedom from distraction or a control of interference).
2. Self-Directed Sensory-Motor Action: This is an alternative means of defining nonverbal working memory and refers chiefly to the use of self-directed visual imagery along with the private rehearsal of visuo-motor actions it permits. While this largely is comprised of visual imagery, or the mind's eye, it also consists of the other senses, such as private hearing to the self, re-tasting, re-smelling, and re-feeling (kinesthetic-proprioceptive experience) to the self. In totality, this EF provides for the conscious, willful re-experiencing of past events that provides a metaphorical Cartesian theater of the mind. Yet, this capacity also eventually comes to include sensory and motor actions to the self that permit an individual to privately practice or re-perform sensory-motor actions to one's self creating a form of mental behavioral simulation.
3. Self-Directed Private Speech: This EF is the alternative explanation for the verbal working memory system to that offered by information processing and traditional cognitive models of EF, such as Baddeley's phonological loop in his model of working memory (Baddeley, 1986; Baddeley & Hitch, 1994). It is based on Vygotsky's theory of the internalization of speech to form the mind's voice used in verbal thinking (see above). Individual's talk to their self to permit not just a private rehearsal of utterances (verbal simulations) but also to self-instruct, self-question, and other verbal means by which language can be used for self-regulation, self-organization, and problem-solving.
4. Self-Directed Emotion/Motivation: This EF is believed to arise from the combination of 1-3 above in which the individual comes to use inhibition, private imagery (and sensing more generally), and private speech to initially inhibit strong emotions and then to down-regulate or otherwise moderate them (Ochsner & Gross, 2005, 2008; Ochsner et al., 2009). They then employ these other EFs to replace the initial strong emotion with alternate emotional responses more consistent with social demands and the individual's goals and longer-term welfare. Because emotions are motivational states, this EF also provides for the capacity for self-motivation – the drive states needed to initiate and sustain action toward the future. Recent research in neuro-imaging and in developing rating scales of EF suggest that the emotion regulation and motivation regulation aspects of this unit may be partially separable both neuro-anatomically (Murray, 2007; Rushworth et al., 2007) and behaviorally (Barkley, 2010c).
5. Self-Directed Play (Reconstitution): This EF is an alternative to the planning, problem solving, and innovative (generativity/fluency) components noted in other views of EF. It is hypothesized to be founded on the development of play and pretense in childhood (Carruthers, 2002) and is seen as essentially a two-step activity. The first is analysis, or taking apart features of the environment and one's own prior behavior toward it. This is followed by synthesis or the recombination of components of the environment and behavioral structures into novel combinations. These novel combinations can then be tested out against a criterion, such as a problem or goal, for their likely effectiveness in over-coming obstacles to goal attainment. While this begins as observable manual play, it progresses like the other EF components to being turned on the self and internalized as a form of private mental play. This permits both the manipulation of mentally represented information about the environment and about prior behavioral structures so as to yield new combinations that can serve as options for problem solving toward goals. It is possible that this component can be subdivided into both nonverbal and verbal modules that provide for fluency and generativity in each of these forms of behavior. Nonverbal, verbal, and action fluencies are believed to arise from this EF.
6. Self-Directed Attention – Self-Awareness: It has become increasingly clear since the foregoing original model of EF was first proposed that a sixth unit may be distinguished from these five. Originally, it was housed within the second module above dealing with self-directed sensory-motor actions, particularly imagery. Yet it is so important as the starting point for EF, has been acknowledged so in prior reviews of PFC functions as the pinnacle EF or central executive, and may arise out of more than just self-directed sensing as to probably warrant being a distinct component of this model. This is the self-direction of attention that comes to create self-awareness. Though listed here as the last component of the model for explication's sake, it likely is the first to arise in development and may well be the most important; out of this module arises self-consciousness or self-awareness that must be seen as a precursor to all other forms of self-regulation set forth above. It is here that the individual becomes conscious or aware of the entirety of their internal and external states, drives, wants, and actions and so achieves an organized, integrated unity or sense of self. As noted previously, this is the prime candidate to be the central executive.
Noteworthy here in discussing self-awareness and self-monitoring is that it does not lead to clairvoyance or a miraculous capacity to actually see the future as it will eventually transpire as may be implied in concepts such as foresight or autonoetic awareness (awareness of self over time). It is instead a means for monitoring and evaluating what is initially automatic behavior as it may occur at the pre-executive level described above. Once such automatic behavior is then detected as being inconsistent with or ineffective at attaining a longer-term goal, self-awareness leads the individual to evaluate the situation and error that has transpired for clues it may yield. From this re-visualizing (and resensing more generally) of the event it enacts a series of steps to adjust such behavior the next time this situation arises so that it better conforms to the path needed to attain the goal, as in feedback loop theory discussed earlier. This is hindsight. The individual is looking back or resensing and even reverbalizing past events and their responses to them for evidence of a pattern that can be identified to guide what re-adjustments may subsequently need to be made to behavior in the next encounter. This leads to an expectation of what to do on the next encounter with that situation. If errors or mistakes have occurred relative to a longer-term goal (value), the sequence is flagged, re-appraised, and a new hypothetical plan or behavioral adjustment is constructed and stored for enactment when the event is next detected. That new contingency will then be used in the next encounter with that situation. Hindsight thereby leads to foresight – such foresight being the individual's expectation or best guess as to what to expect on the next encounter, and even when to expect it, and then what may be better to try on the next encounter with that event. With repeated experiences with the situation, the individual may have fine-tuned their response to dealing with it as well as their expectations as to when it is most likely to occur and thus appear to others to act as if with clairvoyance. However, it is the hard-won knowledge from hindsight that has created it. On encountering a novel situation, the individual has no idea what the future may hold in store. However, he can activate his hindsight of experiences with previous similar situations for clues and patterns that may give them ideas as to what may be best to enact in this situation. Yet it will be the experience of that action that the individual then uses to re-adjust their subsequent behavior.
For instance, when children talk to themselves while performing academic work, such as math problems on paper, such self-talk does not seem to initially benefit the current performance with that problem. Instead, it correlates with improvement in subsequent encounters with that or similar problems (Diaz & Berk, 1992). In short, foresight emerges only as a function of self-monitoring of ongoing and largely automatic activities that can be mentally re-experienced for further analysis (hindsight) when mistakes have been detected out of which a plan for adjusting behavior in the next encounter is constructed. Foresight is the product of that analysis and the hypothetical action proposed for use on encountering that or similar situations again. The human will may therefore be not so much an agent that initiates actions on first encounters with novel situations for which one has no past experience but one that re-analyzes and then re-adjusts subsequent behavior so that it may be more adaptive or effective upon subsequent re-encounters with those events.
To summarize, the EFs at this level arise as a consequence of the individual turning upon themselves certain pre-executive level functions. By self-directing them, the person creates a capacity for self-awareness and self-modification and regulation over time (future-directed, goal-focused, cross-temporal actions). Yet it is not just self-directed action that emerges here, but also a developmental internalization or privatization of those self-directed actions from others that is noteworthy. Why it would be necessary to keep one's self-directed actions private from others is an interesting question; one that may be answered at the next level of EF where imitation and vicarious learning arise and thereby permit others to essentially steal or plagiarize one's behavioral actions and innovations. Thus, two key factors distinguish this level of EF from the non-EF level – the self-direction of human actions and the eventual privatization of them across development to become wholly internal, mental, or "cognitive" in form.
This level makes plain what is contained in the traditional phrase of "those neuro-cognitive processes" that support goal-directed behavior and problem-solving used so often by others to define EF. It makes clear the nature of the constructs placed under the umbrella term or meta-construct of EF at this cognitive level as described by others. They are referred to as processes because they are not ends in themselves but means to higher order ends that are to be found at upper levels (outer spheres or zones) of EF activity in daily human life. They are considered instrumental as they are necessary for the use of EF in those daily life activities but are not sufficient alone for those higher levels to exist. And they are also called self-directed for the obvious reason that it is the self-direction of pre-EF functions that defines an executive from a non-executive function or process. It is unlikely that this is the level of EF that served as the basis for the evolution of EF, given that many other species eek out their survival without an EF system. More likely, natural selection was likely acting on the next higher levels of human behavior and social activities that drove the evolution of this instrumental level so as to meet those challenges that arose at this higher levels.
This necessarily brief review demonstrates that the beginning of the effort to understand the nature of human EF began as a commendable effort to understand the functions of the PFC. That effort originated in the initial attempt to catalog the various functions of the PFC from studies of animal and human PFC injuries. However, once the term EF was pinned to the functions in this catalog in the absence of any operational definition of that term, this history reveals as well a striking heterogeneity to subsequent definitions of the term EF and the constructs believed to comprise it. It is not too strong to conclude that the field of EF as it now stands contains a veritable dog's breakfast of diverse definitions and concepts that continue to hinder further advances in this field of inquiry. If EF is to be a useful construct or umbrella construct, it must stand on its own conceptual legs at the neuropsychological level of analysis independent of any linkage it may have to the PFC at the neurological level of analysis. Significant problems abound in these conceptualizations of EF at the neuropsychological level that will be discussed in the next section.
Table 1 - A Chronological Sampling of Definitions of EF
Many previous writers on the topic of executive functioning (EF) have noted some very troubling issues in prior efforts to conceptualize and evaluate EF that deserve notice (Barkley, 1997b, 2001; Denckla, 1996; Dimond, 1980; Eslinger, 1996; Hayes et al., 1996). Space does not permit a detailed analysis – definition by definition – of all of the difficulties evident in prior efforts to define this construct. However, the major problems are worth some mention here so as to show the continuing need to address these problems while trying to integrate the features common to many of these definitions into a better theory of EF.
The first of these problems was evident in Chapter 1 – there is little, if any, consensus definition of EF or of its components in the field of neuropsychology. For instance, Eslinger (1996) described a conference (which I attended) held in January 1994 at the National Institute of Child Health and Human Development (Washington, DC) in which 10 experts in EF were asked to generate terms that would be considered executive functions – they came up with 33! Little wonder then that the field has made little headway in its development of useful theories of EF in the past 16 years. The greatest agreement (endorsed by 40% or more of the participants) existed for the following six: self-regulation, sequencing of behavior, flexibility, response inhibition, planning, and organization of behavior. It is unlikely to do so in the future so long as so many sub-constructs are specified as being "those neuropsychological processes" that support goal directed behavior.
This was evident in the recent review of the development of EF by Best, Miller, and Jones (2009) where at least 15 components were found in contemporary research on EF. Among these, Best et al. (2009) settled on the most important four for their review. They argued that these components likely develop at different rates and probably in the following sequence: (1) inhibition, (2) working memory, (3) shifting, and (4) planning (which includes problem-solving). Although these EFs are often significantly correlated, supporting the view of some unity among them, the relationships are usually modest, supporting the view that there is also diversity in the construct of EF (Miyake et al., 2000). These different components also have different developmental trajectories, also supporting the diversity view (Best et al., 2009; Huizinga, Dolan, & van der Molen, 2006).
Eslinger (1996) went on to say that the vast majority of views of EF included stopping the initial response, delaying/deferring it (inhibition), and using mental representation of information (and manipulation of it) that will be used to construct and subsequently guide the motor responses (working memory). This is very similar to the two-factor model of EF of Roberts and Pennington (1996) discussed earlier. Eslinger (1996) therefore argues that while EF may include more components than just these constructs of inhibition and working memory, these two are "the gatekeepers" for the mechanisms of problem-solving, skillful use of strategies, and flexibility – the remaining EFs in his view. "In addition to the gate-keeping functions, the working memory mechanism mentally defines the field of play so that actions can be memory-guided rather than sensory-guided."(p.382; Eslinger, 1996).
Apart from the fact that the EFs can be largely (though not entirely) localized to various regions of the prefrontal cortex (PFC), just what makes these mental capacities or modules an EF at the neuropsychological level of definition and analysis? Are we to be stuck with reductionism to the neuro-anatomical level as the only means of defining what constitutes EF? Lacking throughout all of the discussion in Section 1 since Pribram first argued for the concept of EF is a specific or operational definition of an executive program or function that can be applied in research, either for the selection of a method to assess it or for identifying those functions that fall under its conceptual jurisdiction. In short, just what is the conceptual essence of an EF that one could use to distinguish it from a non-EF? The answer is frequently sidestepped by listing "those neuropsychological processes" believed to be subsumed under the "umbrella" term thereby bypassing the need to define the umbrella or meta-construct.
Are the executive functions merely any and all mental processes needed for goal-directed action? If so, then the primary senses, motor movements, attention, memory, arousal, primitive appetitive centers, emotion, and even basic brain functions pertaining to monitoring and sustaining the body are EFs because goal-directed behavior is impossible without them. Yet few theorists would identify these as EF, placing them more often at the non-EF or pre-EF levels in any hierarchy of brain function (Fuster, 1997; Luria, 1966; Stuss & Benson, 1986). Why? What defines the boundary between them and what can be considered EF? This vague, mongrel view of EF will not do if advances in understanding EF at the neuropsychological level are to continue. Hence, our first problem with the concept of EF is that we lack a more precise and operational definition of it that can serve as a guide to making such distinctions between what mental functions are and are not EF.
Dimond (1980) and later Lezak (1995) were correct, I believe, in noting several additional problems that have continued to be ignored by many in the subsequent history of EF to the present time. One of these is the relative sparse recognition in the history of EF of its importance for social functioning and effectiveness, or what Dimond called our social intelligence. Ciairano et al. (2007) at least acknowledged the importance of EF in cooperative social behavior. Eslinger (1996) especially noted the importance of EF in social functioning. Specifically, Dimond referred to ". . . the capacity to respond to appropriate social patterns, to regulate social life and to integrate adequately and successfully with others" that was so important about PFC functioning (p. 510; Dimond, 1980). There is a striking social pathology associated with PFC damage, Dimond (1980) argued, that goes largely unappreciated in efforts to describe the major functions of the PFC. Perhaps this is owing in large part to the fact that clinicians and neuroscientists nearly always study such patients in isolation (individually) and in relatively short periods of time (a few hours at most at a time) in unnatural settings (clinics and labs) and with EF measures that are largely "cold" cognitive in nature that would miss this aspect of functioning or detect only the smallest instances of its degradation. Dimond (1980) makes a special point of noting the hundreds of cases of PFC-injury that did not manifest many of the changes in cold cognition or mental functioning attributed to this region by others except for marked changes in planning and social functioning (pp. 505-508).
" . . . as one of its major functions, the frontal lobe bears responsibility for administering the code by which the patterns of social behavior are put into operation and by which the individual integrates and regulates its conduct in respect of that of other individuals. We postulated that there is a special form of social intelligence by which the organism maintains the running, changing stream of social relationships and that the frontal lobes bear important, if not unique, responsibility for this." (p. 507)."
Eslinger (1996) in particular would later take up this call for the importance of EF (the PFC) in managing the social conduct of the individual. He argued that EF contains "social executors" that each serve certain social functions: (1) social self-regulation: processes needed to manage the initiation, rate, intensity, and duration of social interactions; (2) social self-awareness: knowledge and insight about oneself and the impact of one's behavior on others in social settings; (3) social sensitivity: the ability to understand another's perspective, point of view, or emotional state [similar to empathy]; and (4) social salience: regulate somatic and emotional states that impart a sense of meaningfulness to social situations and to specific individuals within that situation. (p. 390). The latter is clearly related to Damasio's (1994) idea of a somatic marker system that imparts the motivational/emotional salience and valence to information being held in mind that concerns plans and goals. Eslinger (1996) views the social disability arising from EF impairment as frequently being the most distinctive feature beyond just the cold cognitive impairments. He states "Yet there is no comprehensive model of executive function that addresses the interrelationship of cognitive and social aspects of behavior, including the various impairments that can occur." (p. 389). He lists the following as some of the social deficits arising from PFC (hence EF) damage – demanding and self-centered behavior, lack of social tact and restraint, impulsive speech and actions, disinhibition (of immediate self-interests), apathy and indifference, and lack of empathy, among others.
Fuster (1997) also took note of the disturbances that are typically evident in the social conduct of the individual, which he largely attributed to the emotional disturbances (apathy, euphoria, depression) and the disinhibition of basic drives (sexual, appetitive). But given his emphasis of the cross-temporal organization of behavior as a central function of the PFC, he should have also noted the fact that social skills, ethics and morality, law-abidance, reciprocal exchange, and cooperation all hinge on the capacity to conceive of the longer-term consequences of one's actions for one's self and for others (Hazlitt, 1944/2008, 1964/1998). If a patient loses this capacity for conceptualizing the longer-term (the temporal integration of action) then all of these social behaviors would be subject to serious disruption and the individual would fall back to acting from habit and from immediate self-interest.
Absent these rare voices concerning the importance of EF (the PFC) for social behavior, one would have thought that the major deficits suffered by those with PFC injuries were largely cognitive or information processing in form. It is as if what Phinnaes Gage (Harlow, 1848) was most disabled by after his PFC injury was an inability to sort cards into categories by their color, shape, or number, to inhibit responding to an X on a computer screen, or to a recite a string of digits backward, to name just a few of the purely cognitive demands assessed in the typical EF test battery. Anyone who has spent any time with patients with PFC injuries or spoken with their family members would find such cognitive deficits to be trivial in comparison to the major social impairments arising from such injuries. From this clinical (and familial) perspective, the view of EF offered by cognitive psychology or those wedded to EF test batteries is not worth having.
It is axiomatic that we do not live alone – humans are a group-living social species. When we engage in EF, we do so not just aware of our long-term self-interests but with an awareness of and in the context of other self-interested self-regulating agents with whom we interact. EF occurs not only across time toward future goals but usually among social others. EF is instrumental in the ability of the self-interested individual to self-organize, self-select, self-initiate, self-sustain, and self-problem solve in maintaining goal-directed actions over time in the context of other self-interested goal-directed self-regulators. More than just losing sight of the social adaptive problems the EFs likely evolved to solve (Barkley, 2001), most modern writers on EF have lost sight as well of the fact that group living self-regulating agents must not only be aware of what they are doing with regard to themselves but with regard to others in their social network and what those others are likely to do in return. Eslinger (1996) is one of only a few to take note of the importance of social self-awareness in understanding EF. This is the essence of morality or ethical conduct (as both he and I have noted; Barkley, 1997b). Not just modern writers but contemporary clinical tests of EF both miss the critical social nature of EF. Their contents are devoid of social relevance and context. This is an oversight I believe fatal to the utility of the tests of EF in predicting EF in major life activities and our ability to understand its evolutionary history and significance in daily human affairs.
I have elsewhere (Barkley, 2001) argued that virtually all of the efforts to understand the EF system and its components have ignored their likely evolutionary origins and purposes. This is undoubtedly due in part to the legacy of adopting cold cognitive tests and their constructs in trying to study EF. However, neuropsychology is a subspecialty of biology (neurology) as much as of psychology. The governing paradigm in biology is evolution. Yet one is hard pressed to find any mention of it in any treatise on EF by neuropsychologists, except for Dimond (1980) and more recently trade books by Gazzaniga (1998).
The theory of evolution provides a means by which one can understand the functional mechanisms that species have evolved to deal with problems they encounter in their environment – these functional mechanisms are their adaptations. The EF system is a complex functional mechanism that seems as if it were designed for a purpose – it is an adaptation that has evolved to solve a problem or set of problems faced by species that possess it. This is not a non sequitur – the adaptive problem that the EFs may have evolved to solve likely resides in the context of social problem solving. There is a daily need to look ahead and anticipate what others are likely to do in the context of pursuing one's self-interests. We can rightly ask what more specific adaptive problems the EFs evolved to solve in the environmental niche in which humans live. It surely was not sorting cards. Given that the vast majority of species do not possess this adaptation, it is highly unlikely to be necessary for surviving and reproducing on this planet. If it were, many species would have converged on it as a great means of addressing an adaptive problem in coping with the physical environment, such as has happened with the repeated evolution of eyes.
The answer may come from inspecting which species have rudimentary prototypes of EF. Chimpanzees and dolphins seem to have a nonverbal working memory system, as do some species of monkeys, though far less developed. One thing that these species have in common is that they are social creatures. In particular, unlike some group living species of mammals and insects, the former species live in groups with individuals to whom they are not strongly genetically related. Where genetic relationships are high, group living and mutual self-interest can arise by genetic (natural) selection as members of the group are virtual clones of each other and thus have a highly shared genetic self-interest. However, social primates often live in groups with others to whom they may be only modestly genetically related or not at all. The particular behaviors known to exist in the social primates (and dolphins) that deserve consideration as possible reasons for EF are reciprocal exchange (trading behavior, and especially delayed exchanges), social competition, social cooperation or mutualism (social symbiosis), and the proto-ethics and morality that exist to facilitate it. Both delayed reciprocal exchange (giving up a resource now to be repaid later) and mutualism (acting together to achieve goals not possible by an individual) require a sense of time, a means of evaluating the discount of delayed payments or other benefits, and a means of subordinating immediate self-interests to future benefits. Without a capacity to conceive of the longer term, these volitional forms of behavior are not possible. As already noted above, it is precisely such behavior that is grossly impaired in individuals with damage to the PFC.
The species that have a proto-EF system also engage to a certain extent in imitation learning (a form of experiential or behavioral plagiarism). They possess a mirror neuronal system in the PFC that is highly specialized for this purpose. In humans, in particular, there is a dominant or prepotent response to overtly imitate another's actions and it must be actively inhibited from being publicly expressed – the neuronal firing patterns that match the actions being imitated are activated but its release to the musculo-skeletal system is inhibited. This instinct to imitate the actions of another can be partially disinhibited when working memory load increases because that increasing demand on the EF system undermines the executive inhibition of the habit of spontaneous imitation (van Leeuwen, van Baaren, Martin, Dijksterhuis, & Bekkering, 2009). It is also likely to be disinhibited when the PFC is damaged (Luria, 1966).
The capacity to use another's experiences for one's own self-improvement is a tremendously useful adaptation among social species in which members compete against each other for resources. Humans also take imitation to an even higher level, which is vicariously learning or the capacity to do the opposite of what one has seen another do. Vicarious learning is a particularly useful adaptation when it comes to learning from the mistakes made by others, some of which can be injurious or even lethal. It is axiomatic that more learning occurs in response to errors than successes. This must be immeasurably more so if one can profit from the mistakes made by others by observing their actions and consequences and then suppressing their own behavioral predispositions to behave in such a fashion in response to errors made by another. The inability to act in opposition to information and actions perceived in the sensory fields is a classic symptom of PFC damage (Luria, 1966; Stuss & Benson, 1986). The capacity to mentally represent information (working memory) allows an individual to wrest control of moment-to-moment behaving and even to act in opposition to what is seen.
There may be other social problems that the EF system has evolved to solve. These may include theory of mind (anticipating that another also has a mind and especially an EF system and acting accordingly) and empathy (Grattan, Bloomer, Archambault, & Eslinger, 1994). They may even be facilitated by covert imitation. But those functions of delayed reciprocal exchange, social competition, social cooperation, and imitation and vicarious learning may have been the initial ones that kicked off the expansion of the PFC in primates and especially humans. Only the first would be needed to veer human evolution down the path to the others, I believe (Barkley, 1997b, 2001). They are well worth our consideration in understanding EF. Performing a digit span backward task is trivial in comparison and is surely not the adaptive problem the EF system evolved to solve.
The next important problem with prior conceptualizations of EF, again noted by Dimond (1980) and later others (Hayes et al., 1996), is attributing "a central executive" to the PFC. This was the initial mistake made by Pribram (1973) and it has been repeated since that first appearance of the term executive as applied to the functions of the PFC. Saying that the PFC is the brain's executive installs a dues ex machina, a homunculus of the mind, or marionette operator (Grafman, 1995) into the PFC that serves to explain nothing and will eventually require its own explanation. Saying that the PFC is the brain's central executive merely begs the issue of just who or what is this wizard behind the curtain that is pulling all these levers in managing the lower level non-executive brain systems so as to direct behavior across time toward future goals. Just who or what is even choosing these goals and for whom are they being chosen? It is surely not some little CEO of a large corporation or a symphony conductor installed in the brain as suggested in the analogies so often used as exemplars of EF in the trade literature. Yet most models of EF include some thinly veiled reference to some sort of "mini-me" homunculus that is doing our bidding, as Hayes et al. (1996) noted.
Since its inception by Pribram more than 40 years ago, this conceptual can of who or what is the central executive has been kicked down the road incessantly. But it must eventually be addressed. One can temporarily side-step this issue as is evident in decisions by working memory scientists (Baddeley, 1986; Goldman-Rakic, 1995) to intentionally ignore the nature of the "central executive." Instead, they focus their research on the subordinate working memory systems that serve to hold in mind the information and goals that the central executive has chosen. Or the central executive is simply inferred from the shared variance between nonverbal and verbal working memory tests without actually defining it and assessing it directly (Rapport, Alderson, Kofler, Sarver, Bolden, & Sims, 2008). Understanding the nature of working memory is clearly a laudable and worthy research goal. However, it does not get us very far in understanding the entity to which the working memory systems are said to be slave units. Who or what is determining the contents of working memory and the goals that they serve? Strikingly absent from most views of EF other than that of Stuss and Benson (1986) is a role of The Self in these models.
In my view, the conscious self IS the central executive. Over development, each of us develops a conscious sense of self and it is this conscious self that is serving as the executive. It is through our self-awareness/self-consciousness that our values and wants are consciously known to us, our goals (values or what we wish to pursue) are chosen by us, and the strategies that we will employ in these pursuits are selected by us. Who chooses? I do. What is to be pursued? What I choose to do. How is it to be pursued? The way I decide to do so. The "I" has been almost entirely jettisoned from cognitive theories of EF replaced by some unknown, undefined central executive. This conscious capacity to consider who and what we are, what we will value, and how and when it will be pursued originates in our self-awareness. It is the seat of human free will as philosophy has noted (our freedom to choose among various goals over various time periods and the means to attain those goals). Stuss and Benson (1986) were absolutely correct in making this mental capacity the apex of their pyramid of the EF system and its components.
Freedom here does not mean random or uncorrelated decision making between values (goals) and their means-ends. Freedom or free will is a conscious generation of and consideration of the variety of options available to that individual over the longer-term as capable of being conceived by the individual and the selection of which goals to pursue, how to pursue them, and when to do so. This active agency of the self exists in philosophy but seems lost to or intentionally avoided by the field of neuropsychology. Perhaps this is because it is seen as unscientific or unmeasurable. But it is neither. The view neuropsychology offers of humanity instead is frankly not worth having – an Orwellian automaton of an information processor without a sense of self.
Alternatively, constructs can be proposed in a scientific analysis of an issue on the basis of reason, experience, and logic that cannot yet be measured objectively at the moment. Objective measurement is not a precondition of scientific theorizing, just the eventual possibility of testability of the proposal. We know that self-awareness is a brain function (Stuss & Benson, 1986) and we have ample evidence that this sense of ourselves as an active, thinking-choosing agent can be diminished by brain injuries, especially to the PFC. EF in modern neuropsychology however is often spoken of in the third person and even then not as a part of a living self-conscious entity, as seen in statements such as this one about the nature of EF: "it monitors and controls all the steps necessary for a correct solution" (p. 242, Borkowski & Burke, 1996). It?
We should immediately recognize that the "it" here is actually the "I." Efforts to strip the self and self-awareness from EF are unnecessarily sterile of what every human accepts as axiomatic and as common sense: I am the agent consciously deciding what it is that I will do. And others hold that "I" accountable for its actions precisely for this reason. A software program in a computer is incapable of being held legally accountable for its choice of actions but a human can and should be held so accountable. One chooses what he or she will do and ought to do using one's self-awareness and sense of the future – the longer-term consequences that are likely to ensue for one's self and for others given the various choices under consideration. It is time to return the self to the construct of EF.
A further problem with most definitions of EF is the relative dearth of attention given to emotional and motivational aspects of self-regulation. While this is certainly discussed by Luria (1966) and others who described the consequences of frontal lobe injuries in humans and primates (Damasio, 1994; Dimond, 1980; Fuster, 1997; Stuss & Benson, 1986) it has been largely ignored in most other conceptualizations of EF in the past 20 years. This is particularly so in accounts of EF using cognitive psychology and information processing models. Exceptions have been Fuster's theory of cross-temporal synthesis (1997), Damasio's (1994) somatic marker theory, Stuss and Benson's (1986) hierarchical model, and my own hybrid model of EF (Barkley, 1997a, 1997b). None are based on the computer metaphor of brain functioning that underlies information processing models of EF. Perhaps this is because computers do not have emotions that need self-regulating and do not have to self-motivate.
The neglect of emotion may also stem from the inherently greater difficulty in measuring emotional and motivational states relative to the enormous number of tests available for assessing the more "cognitive" features of EF, such as working memory. Emotions are motivational states that undoubtedly play an important role in the evaluation and determination of one's means (actions) and ends (goals) and their social appropriateness (Damasio, 1994). They will also contribute the drive, willpower, or self-motivation that will be needed to achieve them (Barkley, 1997b; Fuster, 1997; Stuss & Benson, 1986). The supposedly "cool" EF brain networks, such as working memory, planning, problem-solving, and foresight, may provide for the "what, where, and when" of goal-directed action, but it is the "hot" EF brain network (Castellanos et al., 2006; Nigg & Casey, 2005) that provides the "why" or basis for choosing to pursue that goal in the first place and the motivation that will be needed to get there.
The foregoing discussion suggests yet a further problem with contemporary views of EF, and especially those predicated on information-processing models of brain functioning. Using the computer as a metaphor for brain functioning has undoubtedly been of value in efforts to advance the understanding of neural circuitry and its likely functions. But it has its limits for brains and it is, after all, just a metaphor not an analogy. Appreciating some of the major differences between them is important. First, computers are designed, human brains evolved. Hence, the architecture and functioning of each are likely to be quite different. Engineers designing computers can determine the most efficient and effective designs for both hardware and software to achieve the intended purposes to which computers are used. Natural selection, acting on brains, has no such plans and foresight to use in its sculpting of the brain. Consequently, a computer may be a marvel of efficient design; a brain is likely to be a veritable Rube Goldberg device of adaptations cobbled together from what had been available for other functions previously but that may be diverted to another function under a change in environmental pressure (adaptive problem). Evolution can only work on what was previously available and gradually tweak previous mechanisms or adaptations for use in the new function to which it is being put. As such, it is a patchwork of kludges or solutions arising out of whatever pieces (functions) were around at the time, so to speak. We must therefore be prepared for the fact that the human brain may be a mixture of older or vestigial adaptive functions that may be less useful or even disadvantageous in modern environments mixed up with ones that may even now still be in the process of evolving toward greater efficiency and effectiveness. Not all of the functions of the PFC are likely to be presently adaptive or useful and others may be frankly maladaptive in modern, industrialized environments.
A second major distinction is that computers are passive – human brains are not. The computer metaphor portrays the brain as if it were software and hardware; given a certain input, this "computer" moves the information through various stages of processing to produce the output that we see virtually automatically, like an automaton, industrial robot, or artificial intelligence creation. This is a very passive view of the organism, devoid of what makes living things unique – they are self-interested agents with basic motives pre-installed that serve to sustain their own life (appetitive and defensive/protective motives) and to reproduce (pass along their genes) into the next generation (sexuality and competition for mates). This blindness to self-interestedness and its motives is a glaring deficiency in these models. A computer is not a self-interested, self-motivating, self-regulating entity – humans are.
Another feature of animal life is locomotion – animals move and act under their own power and must be attentive to refueling and maintaining their vehicles often. Nature does not automatically provide for the life-sustaining needs of a human; that person must interact in such a way with that environment to produce its sustenance. Computers are not self-interested, do not self-assemble, do not compete with other computers for resources or mating rights, and do not concern themselves with the source of their own fuel. Such motivational considerations from biological evolution are absent in the modern concepts of EF. Only Dimond (1980, p. 504) seems to have taken note of them and given them some importance in understanding the losses that occur in PFC damage – our social intelligence that is necessary to insure our survival and reproductive self-interests. Humans have motivations that computers do not.
Some EF perspectives do acknowledge that drive, motivation, and will are prefrontal functions and are a component of or enslaved by EF (Stuss & Benson, 1986). But even these few fail to note that this is the fuel tank of all future-directed action and the EFs that contribute to that action. Humans act and they do so with purpose (intentionality; a future directed stance). Those actions are initiated and sustained by drive, motivation, and will and the self-interested motives to survive, nourish themselves, and reproduce themselves into the next generation. Absent an appreciation for such motives in human action, computer metaphors of EF will prove strikingly sterile and self-limiting in helping us to understand EF or the functions of the PFC.
The sample of definitions and conceptualizations of EF (or the functions of the prefrontal cortex – PFC) discussed in Section 1 makes it clear that, despite the problems noted in Section 2, some common features exist in these efforts that may be useful in understanding the meaning and nature of this concept and formulating a clearer, more specific definition of it. Perhaps, as Dimond (1980) asserted:
"Attempts to describe the functions of the frontal lobe within the framework of a single characteristic unitary defect appear to be doomed to failure from the outset, because the frontal lobe is no small ganglia, no tiny knot of tissue, but a vast corpus the functions of which in all probability are as numerous as the frontal lobes are large. Although we may seek no one basic defect associated with damage to the frontal lobes, that does not preclude the search for broad principles of action upon which the functioning of the frontal lobes may be based." (p. 508-509).
I would concur. The complexity of the PFC, of necessity, will be matched by the complexity of EF that it supports. However, some broad principles can nonetheless be discerned.
It would therefore appear, given the foregoing reasoning, that EF can be defined initially as self-regulation across time for the attainment of one's goals (long-term self-interests) typically in the context of others. As is obvious from that same analysis, EF is not a single construct but an over-arching meta-construct in the service of which are various forms of self-regulation. What binds them all together as being executive in nature is that all are forms of such self-regulation across time to achieve goals in a social group – that is the essential core of the EF construct. It is achieved by multiple, interacting mental modules or neuropsychological capacities (constructs) each of which is a type of self-regulation that contributes to the larger longer term goal of EF (the meta-construct). Disturbances or difficulties in the effective functioning of any one self-regulatory module disturb EF but in a different way from disturbances in other modules. In other words, self-regulation is not one thing but involves multiple self-directed activities.
These activities or mental capacities include self-stopping (inhibition), self-awareness over time (a conscious sense of time, past, and the future – hindsight, foresight), working memory (self-directed sensory-motor action that creates mentally represented information), self-planning, self-problem-solving, and self-monitoring (and shifting as needed), and sustained self-protection of EF from interference (freedom from distraction). Since a problem can be defined as a situation for which one does not have a readily available response, it is by definition a novel situation. Therefore, EF and its associated mental modules will appear to be most needed and most activated when novelty (problems) have been encountered.
To conclude, the commonalities across the history of EF indicate that (a) it is a meta-construct (umbrella term); (b) it can be operationally defined as self-regulation across time for the attainment of one's goals (long-term self-interests) typically in the context of others without any significant loss of the information conveyed by the diversity of definitions used in the prior literature; and (c) its component mental modules (neuropsychological processes) that are needed to engage in such self-regulated behavior across time that is directed at hypothetical social futures include more specific self-regulatory actions. Those self-directed actions consist of inhibition, self-directed sensory-motor actions (foresight, hindsight, sense of time), self-directed attention (self-awareness and self-monitoring), working memory (self-imagery and self-speech, or nonverbal and verbal), planning and problem-solving (self-directed play), self-motivation, and emotional self-regulation. A major advantage in this perspective on EF is that it specifically identifies what makes a human action or mental process executive in nature – an executive act or process is a self-directed activity being used to modify subsequent behavior so as to achieve a change in the delayed (future) consequences for the individual. For instance, orienting attention to an environmental event is not executive in nature; directing one's attention to the self is executive in nature. Speaking to another person is not an executive activity; but speaking to one's self with a series of self-directed instructions or questions is executive in nature. The key to identifying any human activity as being executive in nature is whether or not that activity is being self-directed in an effort to change subsequent behavior so as to attain a goal (a delayed consequence).
The next issue to be considered here is of utmost important to the clinical evaluation of EF. That issue is "What is the best means for detecting or assessing EF deficits?" Dimond argued that more was to be gained from understanding the most important functions of the PFC by using the methods of behavioral ethology (e.g., field observations of the natural behavior of organisms) rather than laboratory/clinical testing. By this, he meant using direct observations of PFC-injured patients in naturalistic settings as they try to go about meeting the demands of major life activities. Such a stance toward the need to observe the consequences of PFC damage in everyday life activities and hence its important functions in more extended, complex, and naturalistic social settings would later be echoed by Grafman (1995), Shallice and Burgess (1991), and others (Alderman, Burgess, Knight, & Henman, 2003; Burgess, 1997). Some of them went on to develop more ethological assessment methods, such as evaluating patients given social tasks that demand "scripts" or sequentially planned and executed nested sets of hierarchically organized behavior concerning a major life activity as a means to evaluate EF in natural settings (Grafman, 1995). Others would develop rating scales of EF in daily life activities (Burgess, 1997; Gioia et al., 2000; Barkley, 2010). Despite such insightful suggestions, neuropsychology has remained largely wedded to the cognitive (laboratory) test battery as its primary if not sole means of evaluating EF. This sole focus on tests of cold cognition as the gold standard for measuring EF has proven to be a major mistake, I believe.
Many tests have been declared to be measures of EF in the neuropsychological literature, too numerous to discuss in any detail here (see the meta-analyses of Frazier, Demareem, & Youngstrom, 2004; Hervey, Epstein, & Curry, 2004; for a lengthy list of those EF tests used just in studies of ADHD; See Lezak, 2004, for a more comprehensive review). One reason for questioning the premise that EF tests are the gold standard for measuring EF is that many of these tests were typically not originally designed to do so (see Lezak, 2004). For instance, continuous performance tests now employed so commonly to evaluate the constructs of both sustained attention and inhibition were originally designed for use in studies of schizophrenia, not EF. The Wisconsin Card Sort Test, that purported pinnacle of EF tests, was originally developed to assess abstract learning and concept formation along with "shift of set" (Lezak, 2004, p. 621). Further illustrating this confusion of the nature of EF tests is that the WCST is at once believed to assess EF while simultaneously being used by others as a measure of a particular type of attention (shifting)(Mirsky, 1996). Nor is it made clear just how abstract learning and concept formation are part of EF when EF is defined as neuro-cognitive processes directed toward a goal. Perhaps the initial discovery by Milner that patients with frontal lobe injuries had deficits on this test automatically made it an EF test because EF has been used as a shorthand phrase for whatever the frontal lobes do (Lezak, 1995). Further undermining the WCST as a putative measure of EF is that some research shows that it is not helpful in distinguishing frontal from nonfrontal injuries (see Lezak, 1995, p. 624; Dodrill, 1997, p. 6). This same problem appears to exist with many other neuropsychological tests believed to evaluate frontal lobe functions (Dodrill, 1997).
Thus, even a cursory glance at the history of EF tests shows that many were originally developed to assess other psychological functions such as attention, memory (both verbal and visuo-spatial), sequencing, abstract reasoning, and language. Others were intended to assess directly response inhibition, planning, and problem-solving without regard to these constructs being involved in EF. Moreover, most do not assess frontal lobe (EF) functions exclusively or differentially (Dodrill, 1997). Once the umbrella term of EF was defined as including these constructs or once it might be shown that deficits on the test were apparently associated with injuries to the PFC, however, these tests were conscripted into being EF tests without regard to whether they actually were sampling the conceptual domain of the construct of EF.
Further supporting these cautionary remarks about EF tests is the current confusion in the very definition of EF as noted in Section 1. EF is quite ambiguously defined and there is little or no consensus among researchers on the precise meaning of the term (Castellanos et al., 2006; Willcutt et al., 2005). Instead, reviews of the research seem to focus more on cataloguing the constructs thought to be subsumed under the term and the tests believed to evaluate those constructs, such as response inhibition, resistance to distraction, working memory, planning and problem-solving, and set shifting (Boonstra, Oosterlaan, Sergeant, & Buitelaar, 2005; Frazier et al., 2004; Hervey et al., 2004; Willcutt et al., 2005). As noted previously, one definition of EF appearing frequently in the literature on ADHD is that given by Welsh and Pennington (1988) but which actually dates earlier to Luria (1966). It refers to those neuro-cognitive processes that are involved in the maintenance of problem-solving toward a goal (Nigg et al., 2005; Willcutt et al., 2005). A comparable definition has appeared in summaries of the primate and human research literature on the functions of the prefrontal cortex in which the cross-temporal organization of behavior toward future goals is seen as the over-arching purpose of the executive functions (Fuster, 1997).
If these are to be the definitions of EF, it is not immediately apparent how traditional tests of EF using such small ascertainment windows to sample behavior in the clinic (typically 5-30 minutes per test) are sampling the cross-temporal nature of EF implied by these definitions. Or if they do so, it is only for exceptionally short temporal durations relative to the hours, days, and weeks over which adults sustain their goal-directed activities. This incredibly brief ascertainment window inherent in EF tests surely must make it difficult if not impossible for EF tests alone to capture the lengthy cross-temporal structures of normal human action. This problem of grossly limited temporal ascertainment by tests alone would insure that they are weakly or unrelated to measures of EF taken in naturalistic settings such as observations over days or by ratings over weeks and months. In contrast, ratings of EF in daily life ascertain behavior across considerably longer periods of time (weeks to months) and therefore better serve as an indicator of cross-temporal behavioral organization and problem-solving toward goals in naturalistic settings than might EF tests.
Also related to this issue of sampling that is problematic for EF tests is their noteworthy inability to evaluate some of the most important features of EF as commented on by some previous authors back to Luria (1966) and even the earliest descriptions of PFC injuries. EF tests simply do not sample many of the capacities believed to be central to the construct of EF. These include the constructs of volition and human will, intentionality or purposiveness, self-awareness (of self, context, and others), and even aspects of planning (foresight, objectivity, choice and comparative judgment, hierarchical structuring) and plan execution (self-motivation, self-monitoring, prolonged resistance to interference by goal-irrelevant events, etc.) as noted by Lezak (1995) (see Chapter 1). Moreover, given the near consensus that EF is self-regulation, it is also not obvious how current EF tests can assess such self-modification for long-term self-interestedness, self-sufficiency, and social independence. Yet these functions mentioned by Lezak (1995) and Eslinger (1996), among others, are inherent in their concept of EF. In addition, if EF evolved for principally social functions, such as reciprocity and social exchange, competition, cooperation, and mutualism more generally, as noted above, the absence of such purposes and motives in traditional EF tests would further assure their severely limited utility in predicting EF in daily life activities and in major domains of adaptive functioning.
Further complicating the ability of EF tests to actually evaluate the construct(s) of EF is both their low internal and low test-retest reliability (Rabbit, 1997). Given that reliability sets a ceiling on the validity of any task, low reliability nearly guarantees low validity. Yet this situation has not troubled most investigators who continue to rely on EF tests as the gold standard of measurement for evaluating this construct or meta-construct.
Yet another reason to be concerned about the reliance on EF tests as the best index of EF is that most EF tasks are not only complex but contaminated, involving multiple cognitive processes many of which are not considered part of EF. Only some of those processes are supposedly reflecting the EF construct that is intended to be sampled (Anderson, 2002; Castellanos et al., 2006). A related concern is that many EF tests are often found to be significantly influenced or contaminated by overall general cognitive ability or level of intelligence (Mahone, Hagelthora, Cutting, Schuerholz, Pelletier, Rawlins, Singer, & Denckla, 2002; Riccio, Hall, Morgan, Hynd, & Gonzalez, 1994). All this makes the results of EF tests difficult to interpret as reflecting unadulterated measures of a particular EF construct. These problems likely explain the findings that statistically removing IQ from relationships between EF tests and observations and ratings of EF in natural settings often reduces any significant relationships to nonsignificant status (Mahone et al., 2002; Mangeot et al., 2002). It may also account for the fact that some of the strongest relationships noted to date have been between EF tests and academic achievement scores (Biederman, Petty, Fried, Black, Faneuil, et al., 2008; Gropper & Tannock, 2009; Thorell, 2007) or self-ratings of academic performance (Ready et al., 2001). Given that both are significantly related to IQ not to mention shared method variance (testing) where academic tests are used, this finding is not surprising.
This problem of conceptual contamination is far less so for EF rating scales whose items have been intentionally selected to directly sample the various behaviors specified in EF constructs (for instance, see Barkley & Murphy, 2010a). These scales or direct observations also have little or no significant relationships with intelligence (Alderman et al., 2003; Barkley & Murphy, 2010a) and so the issue of contamination by general cognitive ability is far less problematic for ratings than for EF tests. Hence, the conceptual or face validity of rating scales may be superior to that of EF tests merely as a consequence of their initial construction. As noted above, many EF tests were not initially designed to measure the construct of EF.
A significant problem for the clinical use of EF tests is that only a minority of patients experiencing frontal lobe injuries or those with ADHD presumed to have a frontal lobe disorder score in the impaired range on these measures. In contrast, the vast majority do on ratings of EF in daily life activities or in direct observations of EF performance in natural settings (Alderman et al., 2003; Barkley & Murphy, 2010a; Burgess, Alderman, Evans, Emslie, & Wilson, 1998; Kertesz, Nadkarni, Davidson, & Thomas, 2000; Mitchell & Miller, 2008; Wood & Liossi, 2006). Given the strong linkage of EF to PFC functioning in the history of EF (see Section 1), it is unlikely that those patients having disorders of the PFC would not likewise show impairment in EF. In ADHD, this situation has led some to argue, however, that ADHD is probably not a disorder of EF given that the majority of cases have no deficits on EF tests (Boonstra et al., 2005; Jonsdottir, Douma, Sergeant, & Scherder, 2006; Willcutt et al., 2005). It is a conclusion much harder to justify logically in patients with demonstrated PFC damage if the PFC is the source of EF.
A further challenge to EF tests as the best means to assess EF comes from the fact that EF tests have very low or no ecological validity. That is, they correlate poorly if at all with ratings of EF in daily life activities in natural settings in adults (Alderman et al., 2003; Bogod, Mateer, & MacDonald, 2003; Burgess et al. , 1998; Chaytor, Schmitter-Edgecombe, & Burr, 2006; Ready et al., 2001; Wood & Liossi, 2006) or children with frontal lobe lesions, traumatic brain injuries (TBI), or other neurological or developmental disorders (Andersonet al., 2002; Mangeot et al., 2002; Vriezen & Pigott, 2002). This is also the case in both adults with ADHD and children with ADHD followed to adulthood (Barkley & Murphy, 2010a; Barkley & Fischer, 2010). The results of these various studies usually reveal that any single EF test shares just 0-10% of its variance with EF ratings. The relationships are frequently not statistically significant. Even the best combination of EF tests shares approximately 12-20% of the variance with EF ratings or observations as reflected in these studies. Yet these two types of measurement are supposed to be measuring the same construct – EF. If IQ is statistically removed from the results, the few significant relationships found in these studies between EF tests and EF ratings may even become non-significant (Mangeot et al., 2002). Something is terribly amiss here if different approaches to measuring the same construct are found to be so poorly related to each other.
Additional evidence for the low ecological validity of EF tests comes from studies in which the performance of frontal lobe injured patients have been directly observed as they go about tasks in daily life. These studies, too, find little or no relationship between impairment in such performances and EF test results (Alderman et al., 2003; Mitchell & Miller, 2008). Here again, EF tests may account for just 9-15% of the variance or less in ratings of adaptive impairment, primarily in work activities (O'Shea, Poz, Michael, Berrios, Evans, & Rubinstein, 2010; Ready et al., 2001; Stavro, Ettenhofer, & Nigg, 2007). In contrast, research has noted moderate relationships between EF ratings and measures of daily adaptive functioning in children with various disorders including traumatic brain injuries (Gilotty et al., 2002; Mangeot et al., 2002) and in adults with ADHD (Biederman, Petty, Fried, Doyle, Mick et al., 2007), those with frontal lobe disorders (Alderman et al., 2003), and college undergraduates (Ready et al., 2001). Furthermore, EF ratings substantially out-predict EF tests in the variance shared with measures of impairments in various major life activities such as occupational functioning, educational history, driving, money management, and criminal conduct (Barkley & Fischer, 2010; Barkley & Murphy, 2010b). The totality of findings to date concerning the relationship of EF tests to EF ratings and of each to impairment in daily life indicates that EF tests are largely not sampling the same constructs as are EF ratings or direct evaluations of EF in daily life (Alderman et al., 2003; Shallice & Burgess, 1991). It also provides a basis for not accepting EF tests as the primary or sole source for establishing the nature of EF deficits in various disorders.
One solution to this has been to try to develop more ecologically valid tests for use in clinics, such as the BADS (Behavioral Assessment of the Dysexecutive Syndrome, Wilson, Alderman, Burgess, Emslie, & Evans, 1996). This has proven only partially successful (see O'Shea et al., 2010). Correlations improve somewhat in normal control groups but not necessarily in disordered samples; even then the shared variance improves to just 16% (O'Shea et al., 2010).
The evidence to date indicates that if assessing how well people do in using EF in their daily life activities is important in clinically evaluating EF, then rating scales assessing EF are superior to EF tests in doing so. Yet the very plethora of studies using tests exclusively to evaluate EF with various patient and general population samples indicates that these serious problems with EF tests have gone largely unknown, unappreciated, or ignored.
This significant failure of EF tests to relate well to EF ratings, daily life activities, or impairment in major domains of life could well indicate that the former are not assessing EF. This seems doubtful given that many of these tests have been shown to index activities in various regions of the PFC that largely underlies EF. And it is surely unlikely to be the case that EF ratings are not actually evaluating EF. After all, their item content has been drafted directly from definitions of EF and from lists of putative EF constructs in the literature as well as from observations and clinical descriptions of patients with PFC lesions believed to manifest the "dysexecutive syndrome" (Burgess et al., 1998; Gioia et al., 2000; Kertesz et al., 2000). Moreover, as noted above, these ratings are substantially related to impairment in various daily life activities and various domains of adaptive functioning, such as work, education, driving, social relationships, self-sufficiency, etc. in which EF would surely be operative. Therefore, the solution to this paradox of why EF tests and EF ratings are so poorly related lies elsewhere rather than in denigrating the EF rating scales.
Most likely, the answer to this paradox lies in the fact that EF is more hierarchically organized than models built entirely on EF tests indicate (Barkley, 2011a). EF tests likely assess the most rudimentary, moment-to-moment, instrumental and cognitive level of EF. But they are very poor at capturing the higher adaptive, tactical, strategic, and principled levels of EF as they are deployed in daily adaptive functioning, human interactions, and socially cooperative and reciprocal activities that play out over much longer spans of time (days, weeks, months, and years)(Barkley, 2011a). It is here, at these higher, more complex, and longer-term levels, that a rating scale of EF can be useful. As noted below, rating scales employ a far longer ascertainment window for capturing summary judgments of behavior over time. They also capture EF symptoms in their important, largely social, contexts via the reports of the respondent and from others who know that person well.
If the purpose of evaluating EF in patients is to render some judgment as to their likelihood of experiencing impairments in major domains of life activities, then EF ratings are far superior in doing so than are EF tests. On the other hand, if the purpose of the evaluation is to assess the most proximal neuropsychological EF activities related to moment-to-moment brain activity, as may be important to do in functional brain imaging studies, then EF tests might be preferable. Yet even that point is arguable in view of recent studies linking neuro-imaging results to traits that were assessed via rating scales (Buckholtz, Treadway, Cowan, Woodward, Li, Ansari et al., 2010). Moreover, such tests can be criticized for stripping out important social and cross-temporal elements to EF that may reduce the validity of the task in sampling any particular EF or its adaptive (evolutionary) purposes.
With these limitations of and issues surrounding the use of rating scales for evaluating human behavior and psychological traits in mind, consider now a list of the advantages of rating scales for clinical and research purposes, again with the caveat that the purpose of the evaluation is the determining factor in the final analysis of what method is best to assess EF. To their considerable credit, rating scales have numerous advantages (see Barkley, 1988; Cairns & Green, 1979):
For these and other reasons, behavior ratings of EF and its deficits in daily life activities have an important role to play in research and clinical practice in various psychological specialties.
Several rating scales of EF now exist that have normative information based on the U.S. population. These include the Behavior Rating Inventory of Executive Functioning (BRIEF) for children (Gioia et al., 2000) and the adult version (Roth et al., 2006) as well as the more recent Deficits in Executive Functioning Scale for adults (DEFS; Barkley, 2011b). Other scales exist for research purposes but have no normative information on U.S. populations. Factor analyses of the rating scales typically identify a different set of EF dimensions than has been previously found using batteries of EF tests. The BRIEF, for instance, yields two empirical (factor based) dimensions: Behavioral Inhibition and Metacognitive. The DEFS yielded five factorial dimensions: Self-Management to Time, Self-Organization and Problem-Solving, Self-Restraint, Self-Motivation, and Self-Regulation of Emotion. Both types of scales have good evidence for their reliability and validity. The DEFS in particular has more than 16 years of research behind its development and has been shown to predict numerous domains of major life activities (Barkley, 2011b).
However, ratings of behavior, including those of EF, have their own inherent disadvantages. These problems with behavior rating scales have been discussed in detail previously (Barkley, 1987; Cairns & Green, 1979) the major points of which will be summarized here.
Rating scales assume that the respondent and examiner share an understanding with regard to the nature of the item being rated and the meaning of the anchor points or potential answers being provided on the scale for responding to that item. One such difficulty therefore can arise in the specificity or clarity of the item to be rated as well as in the answers or anchor points being provided for the rating of the item. How well do the examiner and respondent share an understanding of the items and potential responses? The more precisely the item and its answers can be specified, the greater is the likelihood that the respondent and examiner will share an understanding of the nature of the behavior or trait to be rated and the answers to be provided.
The generality or ambiguity of the item being rated can lead to problems in testing hypotheses that require far more precise measurement than the scale is likely to provide. Such a problem arises as a consequence of the nature of the hypothesis being tested or the purpose of the assessment. To the degree that great precision in measurement is needed to fulfill that purpose or to test that hypothesis, rating scales may be a poor choice of assessment method for the construct of interest. On the other hand, there are many questions or hypotheses in psychology or purposes in clinical evaluations that do not require such a level of precision. Indeed, using excessively precise forms of measurement such as tests or direct observations of behavior is often more expensive and time consuming than was necessary to address the issue when a rating scale would have served just as well at far less cost and time. The potential problem of precision of measurement with a rating scale is only so when judged relative to the hypothesis being tested or intended purpose of the clinical evaluation and not some inherent and absolute flaw of rating scales outright.
Various confounding factors may influence the capacity of the rater to provide an accurate report of the behavior represented in the items on the scale. Level of intelligence, education, emotional status, range of life experiences, prior experience with similar rating scales, and a myriad of other factors have the potential to bias the reports of the rater in ways that may affect the accuracy or validity of the ratings being provided. For instance, adult anxiety may result in an over-reporting of ADHD symptoms by that adult relative to that level reported by someone who knows the patient well using the same scale and the same is true in EF ratings (see Barkley, Knouse, & Murphy, in press). It is conceivable that this represents a true biasing of the rating of the adult such that it is less valid or less accurate than that rating being provided by the collateral. Users of rating scales clearly need to be aware of any findings from research concerning the nature of such biasing effects on the type of rating scale under consideration.
Yet it is also possible that level of anxiety actually does result in a greater degree of ADHD symptoms or, in the present case, ratings of EF deficits in daily life and therefore the rater's report is not inaccurate but simply reflecting this real impact of anxiety on EF deficits. Performance on EF tests can likewise be adversely affected by the presence of other characteristics of the examinee and, as here, may either reflect a bias away from the true or valid level of performance or reflect a truly adverse effect on EF performance that is not a bias at all. Obviously far more research is needed on the issue of the extent to which other features of the rater influence their ratings of EF, whether representing a bias or error in the rating or a true influence on the actual behavior being rated. To the extent that such biases may operate, this will contribute to measurement error reducing the reliability and especially the validity of the rating. However, evidence above indicates that despite such potential biases, rating scales may outperform EF tests in predicting adaptive functioning in daily life activities or impairment in major domains of life (Barkley, 2011b; Barkley & Fischer, in press; Barkley & Murphy, 2010). Ratings therefore may still have considerable value or, in this case, greater ecological validity, even if offering imperfect samples of the behaviors being rated. The question here, then, in the choice of the selection of a measure is the intended purpose for which the measure is being used. There are some purposes that will demand the type of data that can be provided by an EF test or by directly observing the participant's behavior in selected settings. Other purposes can be easily fulfilled by the type of information to be gleaned from the far more cost-effective rating scale.
Rating scales are often criticized for using relatively vague references to frequency of behavior, such as "sometimes," "often," or "very often." To some extent, such criticisms are quite justified if one is interested in very precise fine grain frequency counts of behavior as might be gleaned from direct behavioral observations or test responses, such as on a reaction time task. Such precise frequency counts may be necessary, in fact, to test certain psychological hypotheses. But at the level of clinical practice and judgment and for other research purposes where such precision is often unnecessary and unduly costly and cumbersome, the more general judgments of individuals based upon their own observations of themselves or others have proven to be sufficiently accurate, convenient, inexpensive, and most importantly, reliable, valid, and predictive to be of great utility. Indeed, research comparing the scores derived from clinical tests, such as continuous performance tests or even driving simulators, has often found the test scores to be less predictive of their respective constructs as assessed in natural settings (parent and teacher ratings or inhibition and attention or department of motor vehicle records or reports of others about one's driving, respectively) than are ratings of the individual's test-taking behavior in that same setting completed by the examiner (Barkley, 1991; Barkley, Murphy, DuPaul, & Bush, 2002; Shelton, Barkley, Crosswait, Moorehouse, Fletcher et al., 1998). To reiterate, each approach to measurement has its place depending on the purpose of the evaluation.
Critics of rating scales often charge that the ratings on a scale are more subjective than the responses of the individual to a test. This is a misunderstanding in the meaning of the term subjective, I believe. The answers to rating scale items are not "subjective" and thus cannot be besmirched by this assertion alone. Thoughts, states of mind, and even privately held opinions are subjective as long as they remain in the head/mind and unobservable to others. They cannot be tested against reality for their conformance to it – that is to say their objectivity (Popper, 1979). However, once a thought is expressed publicly in any form (verbal, motor, or emotional behavior) that behavior can be observed by others and recorded, as in the case of a rating scale. The response itself, regardless of the thought it may be linked to, is an item of observable information that can be tested in various ways for its veracity (truth value) and utility. The degree to which the rating actually represents the privately held opinion of the rater may never be known. The degree to which the rating reflects the actual behavior that is to be rated, however, is a different matter and is open to empirical validation. Thus, if a person reports that they have considerable problems remembering things that they have been told to do, it is not of so much concern that their rating be evaluated for how well it captured their private mental state. More important is the issue of how well the rating corresponds to the level of problems the person actually has in remembering such things. The validity of the rating as an index of the problem can be investigated while the validity of the rating as a reflection of a private mental state cannot. However, it is rare that the latter issue is the one under consideration in the research project or clinical evaluation of a patient. It is the former issue that matters most.
To summarize, any charge of subjectivity leveled against interviews and rating scales in an effort to disparage them (and in defense of tests) without reference to further evidence concerning the purpose of the evaluation, the issue under investigation, and the evidence for reliability, validity, and utility of the scale is baseless. The validity of relatively broad human estimates as recorded in structured interviews or rating scales concerning the relative frequency of specific behaviors is itself a form of information that can be observed, recorded, and tested, for its reliability, validity, and utility no different than a response to any test. That is what matters and should be the basis upon which a clinician or scientist chooses a method of measurement and not on some inherent bias in favor of tests over rating scales no matter the issue at hand.
Whether or not the behavior of a participant is recorded by their circling of a response to an item on a rating scale or a button press on a device or a verbal response recorded on some testing answer sheet is a distinction without a difference. All are forms of observable and recordable behavior and the proof of their validity is in the evidence available about them. The knee-jerk and widespread penchant of neuropsychologists to assume that a test is somehow more objective, precise, and therefore useful than a rating scale is a hypothesis; it is not a fact in the bag. The claim deserves to be subjected to testing in research and is not a fact by mere consensus of opinion or proclamation by an authority alone. So far, this assumption of the test as a gold standard of measurement has proven questionable for pursuing certain issues or purposes, such as the prediction of functioning in naturalistic settings or the determination of whether a particular disorder involves deficits in EF, such as ADHD. Clinicians, industrial/organizational psychologists, and even researchers need make no apology for using rating scales to assess certain domains of behavior and functioning instead of some form of recording directly observed behavior or via testing if the evidence shows that those scales have merit for the intended purpose. The available evidence shows that for certain purposes rating scales are acceptable forms of measurement, in this case of EF.
As noted above, numerous problems plague not only the assessment of EF, especially by current psychometric methods, but even the very definition and conceptualization of the construct. How is it that tests of EF that have been used for several decades to presumably evaluate this important domain of human mental functioning are largely unrelated or only weakly related to domains of everyday human activities in which EF would be crucial for effective adaptive functioning? Why do rating scales of EF, often dismissed or demoted as too subjective, actually better predict performance in many domains of adult major life activities than do the tests? It is possible that EF tests are not assessing EF as it is currently conceptualized, or that EF ratings are assessing something other or more than EF. The solution here may not be to continue to pit EF tests against EF ratings as if both evaluated the same level of EF. Instead, it is to understand that EF may need to be conceptualized more broadly than has previously been the case. Such a broadening of conceptual models of EF is also indicated by the extraordinary gap that exists between EF constructs as currently conceptualized and measured by psychometric tests, such as working memory, inhibition, planning, etc., and those domains of human life in which EF is indispensable, such as ethics and morality, occupational functioning and economics, social relationships, antisocial behavior, etc. to name just a few major life activities adversely affected by deficits in EF.
Toward that end, I will soon be publishing a new model of EF known as the extended phenotype theory of EF (Barkley, June 2011; Executive Functioning in Everyday Life: Integration, Extended Phenotype, and Clinical Implications. New York: Guilford.). Borrowing from Richard Dawkins' concept of the extended phenotype in biological evolution (Dawkins, 1982), this model examines the radiating effects of EF mental functions upward and outward from the individual into their physical and especially social ecology using a 6 level hierarchical model (or more aptly, 6 overlapping concentric rings) to conceptualize EF in everyday life. Space permits only a brief summary of the model here. Five parameters are used to create the levels of the model; these parameters increase with PFC maturation and along the posterior-to-anterior axis of the PFC. The parameters are: (1) an increasingly longer time horizon (sense of the future) of mental contemplation; (2) an increasing complexity and hierarchical organization to behaviors needed to bridge the time delays under consideration; (3) an increasing preference for larger delayed over smaller immediate consequences or goals (a decrease in the discounting of delayed consequences); (4) an increasing capacity to initiate and sustain self-motivation in support of such increasingly lengthy chains of behavior; and (5) an increasing size and complexity to the social network being employed to achieve one's goals. As these levels arise, additional mental abilities may be needed at the new level not present at earlier levels, such as imitation and vicariously learning, theory of mind, etc.
Pre-Executive Level: This is the inner most ring (lowest level of the hierarchy) of the phenotype of EF. It comprises all of the non-executive brain functions similar to that same level proposed by Stuss and Benson (1986), such as vision and other sensory perceptual processes, speech and language, memory, motor functioning, etc. This is the level out of which the EF system will be constructed and which will be over-ridden or taken over by the EF system for purposes of goal-directed behavior.
Instrumental – Self-Directed: This is the most basic, primitive, and eventually cognitive level of EF. It is instrumental because it is a means to other ends, not an end in itself. It arises from the individual directing non-EF capacities back on themselves. It is the start of self-regulation – a self-directed action intended to alter subsequent behavior so as to achieve a goal (alter a delayed consequence). As children mature, they develop 5-7 forms of self-directed actions for purposes of self-regulation, as previously described in my earlier model of EF (Barkley, 1997a, 1997b). These are the basic EFs. Attention becomes self-directed to produce self-awareness and monitoring. Restraint and inhibition become self-directed to create executive or volitional inhibition, self-restraint, or self-discipline. Sensory-motor actions become self-directed to achieve sensing to the self, chiefly visual imagery or the mind's eye (forming the basis for nonverbal working memory). Speech becomes self-directed to create verbal self-regulation, or the mind's voice (forming the verbal working memory system). These four are used to create self-directed emotional and motivational states through the use of nonverbal and verbal mental representations achieving both the self-regulation of emotions and self-motivation. Lastly, play moves from physical to symbolic, from outwardly directed at the environment to self-directed and mental, and forms the basis for mental problem-solving (simulation of possible response options). This is the level most likely assessed by EF tests though even then with great limitations (small ascertain windows, etc., as noted above).
Methodical – Self-Reliant: As the instrumental level matures, a higher level or concentric zone emerges in which the individual uses the self-directed EF level for daily adaptive functioning and survival. Unique to this level is that the individual is not just self-directing their actions, but is now organizing the immediate physical surroundings to assist with self-regulation toward goals, self-reliance, and social independence. The individual strings together simple chains of actions (a method) that achieve a relatively near-term goal (self-care, sustenance, and protection).
Tactical – Interactive: At this level, the individual is creating somewhat more complex hierarchies of behavior (sets of methods become a tactic) to achieve somewhat longer term goals than at the previous level. More importantly, at this level, the individual not only organizes their physical surroundings to facilitate self-regulation toward goals, they begin to use other people to achieve goals they may not be able to attain or achieve as efficiently as they could alone. Reciprocity, trading, bartering, and social exchange develop at this level as a means to attain longer-term goals. The social level of EF has now emerged.
Strategic – Cooperative: With further maturation and a corresponding increase in the 5 developmental parameters discussed above, a more complex social level of EF emerges. It is characterized by the individual organizing sets of tactics to form more complex goal directed strategies and by the individual self-organizing and participating in a larger social group in order to achieve goals. It is characterized by social cooperation. The forming of individuals into a small group to achieve goals none can attain alone and to share in the consequences. Such groups form and dissolve as goals are formulated and achieved. Longer term social relationships arise such as friendships as like-minded individuals join together and set out to achieve even longer-term and larger-goals not possible when acting alone or merely through social exchange.
Principled – Communal: At this highest level of the extended phenotype of EF, individuals are following more abstract principles concerning human behavior, ethics, morality, laws, government that pertain to communal existence. Sets of strategies are put together to form principles and that serve to generate the most complex and long-term forms of human actions from abstract principles. Along with this development comes the joining together of individuals for longer terms and in larger groups than occurs at the previous group cooperative level. Communities of like-minded individuals arise in which people not only pursue their long-term self-interests, separately and together with others in cooperative groups, they now look out for each other's longer-term self-interests. Functioning at this level requires the development of social rules, laws, and formal mechanisms for supervision, dispute mediation, punishment of individuals breaking these rules, and community defense (police and military). It is the highest level at which humans self-organize their physical and social environment for the benefit of their long-term welfare and goals.
Injury to the EF system (the PFC) will cause a collapsing downward of these levels of the hierarchy or a collapsing inward of these outwardly radiating concentric zones with the degree of collapse directly dependent on the severity of the injury and specific type of EF affected. EF tests may be assessing the most basic instrumental level of EF and even then, not especially well. EF ratings are assessing higher levels (methodical, tactical, and strategic) and so are closer to the impairments in major life activities likely to be adversely affected by disorders of EF (or injuries to the PFC). Hence, they are more predictive of such impairments than are EF tests.
A number of clinical implications arise from this extended phenotype model of EF as discussed in my new text noted above (Barkley, 2011). Through this model, one can now span the extraordinary conceptual gap between EF tests, like digit span backwards, card sorting, tower building, etc. to human daily adaptive functioning, social exchange and reciprocity, social cooperation and communal arrangements, to ethics, morality, economics, law, and government. The EF system is a human's most essential means of survival. Our theories of EF should reflect this fact. The extended phenotype model appears to do so.
This course has reviewed the history of the concept of executive functioning (EF) showing that it originated in far earlier efforts to understand the functions of the prefrontal cortex. The identification of those functions carried forward into the notion that the PFC is the brain's executive and so these PFC functions became identified as the executive functions. While many attempts have been made to conceptualize and define the construct of EF, most such efforts conclude that EF is more likely a meta-construct under which there exist more circumscribed and specialized functions. What all such functions appear to have in common is their involvement in self-regulation and problem-solving toward goals and the future more generally. In my own theory of EF, the major executive functions are viewed as forms of self-regulation (SR, or self-directed actions that serve to alter subsequent behavior so as to affect the likelihood of occurrence of a future consequence or goal). Thus EF = SR. There are at least 5-7 major classes of such self-directed actions all of which by adulthood work together like a set of mind tools to achieve self-regulation across time toward goals and the future. These self-directed actions (EFs) form the initial instrumental or cognitive level of EF but are not the only level.
A new view of EF as an extended phenotype argues that there exist four additional levels of EF that are not likely to be evaluated by EF tests. These higher levels extend self-regulation outward into the physical and especially social environments and across far longer spans of time than can be captured at the instrumental level of EF typically assess by tests (Barkley, 2011a). As the PFC/EF system matures, additional levels emerge that represent the methodical – self-reliant (adaptive), tactical – interactive, strategic – cooperative, and finally the principled – mutualistic levels (Barkley, 2011a). As discussed here, EF tests at best capture the most primitive, moment-to-moment level of EF as instrumental cognitive functioning but even then are limited by their small ascertainment window, lack of social content, limited behavioral complexity, and failure to include emotional and motivational elements of EF. This explains why EF tests have such poor ecological validity. In contrast, rating scales of EF have considerably greater ecological validity in predicting major life activities given their vastly greater ascertainment window, their basis on observations of behavior in many natural settings, their inclusion of social factors, and their greater capacity to capture emotional and motivational aspects of EF in daily life. Despite their own limitations, EF ratings may be better than EF tests at predicting how well people employ EF in daily life across major domains of adaptive functioning.
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