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Introduction: A History of Executive Functioning as a Theoretical and Clinical Construct

Dr. Sam Goldstein

Executive function (EF) has come to be an umbrella term used for a diversity of hypothesized cognitive processes, including planning, working memory, attention, inhibition, self-monitoring, self-regulation, and initiation carried out by prefrontal areas of the frontal lobes.

Although the concept of EF was first defined in the 1970s, the concept of a control mechanism was discussed as far back as the 1840s. Phineas Gage offers perhaps one of the most fascinating case studies associated with EF. In 1840, as a railroad construction foreman, Phineas was pierced with a large iron rod through his frontal lobe (see Ratiu & Talos, 2004 ). This accident destroyed a majority of his left frontal lobe. Phineas survived and after a period of recovery changes in Phineas’ behavior and personality became apparent. Phineas was described as “disinhibited” or “hyperactive,” which suggested a lack of inhibition often found in those with damage to the prefrontal cortex (Pribram, 1973). This case and others prompted early brain researchers to further investigate the role of the frontal lobes and the concept of executive function.

By the 1950s, psychologists and neuroscientists became more interested in understanding the role of the prefrontal cortex in intelligent behavior. British psychologist Donald Broadbent (1953) described differences between automatic and controlled processes. This distinction was further elaborated by Shifrin and Schneider ( 1977 ). These authors introduced the notion of selective attention to which EF is closely related. In 1975, psychologist Michael Posner coined the term “cognitive control” in a book chapter titled “Attention and Cognitive Control.” Posner proposed that there is a separate executive branch of the attentional system responsible for focusing attention on selected aspects of the environment. Alan Baddeley proposed a similar system as part of his model of working memory, arguing there must be a component which he referred to as the “central executive” allowing information to be manipulated in short-term memory. Shallice ( 1988 ) also suggested that attention is regulated by a “supervisory system which can over-ride automatic responses in favor of scheduling behavior on the basis of plans or intentions.” Consensus slowly emerged that this control system is housed in the most anterior portion of the brain, the prefrontal cortex.

Pribram (1973) was one of the first to use the term “executive” when discussing matters of prefrontal cortex functioning. Since then at least 30 or more constructs have been included under the umbrella term, EF, making the concept hard to operationally define. Many authors have made attempts to define the concept of executive function using models that range from one to multiple components. Lezak (1995) suggested that EFs consisted of components related to volition, planning, purposeful action, and effective performance. It has been hypothesized that each component involves a distinct set of related behaviors. Reynolds and Horton ( 2006 ) suggested that EFs are distinct from general knowledge. They suggest that executive functions represent the capacity to plan, to do things, and to perform adaptive actions, while general knowledge related to the retention of an organized set of objective facts. They further hypothesized that EF involves decision making, planning actions, and generating novel motor outputs adapted to external demands rather than the passive retention of information. Naglieri and Goldstein ( 2013 ) based their view of the behavioral aspects of executive function on a large national study of children. They suggest that executive function is best represented as a single phenomena, conceptualized as the efficiency with which individuals go about acquiring knowledge as well as how well problems can be solved across nine areas (attention, emotion regulation, flexibility, inhibitory control, initiation, organization, planning, self-monitoring, and working memory).

A Review of EF Definitions

Anderson (2002 ): “Processes associated with EF are numerous, but the principal elements include anticipation, goal selection, planning, initiation of activity, self-regulation, mental flexibility, deployment of attention, and utilization of feedback.” (p. 71)

Banich ( 2009 ): … “providing resistance to information that is distracting or task irrelevant, switching behavior task goals, utilizing relevant information in support of decision making, categorizing or otherwise abstracting common elements across items, and handling novel information or situations.” (p. 89)

Barkley ( 2011a ): “EF is thus a self-directed set of actions intended to alter a delayed (future) outcome (attain a goal for instance).” (p. 11)

Baron (2004): “Executive functioning skills “allow an individual to perceive stimuli from his or her environment, respond adaptively, flexibly change direction, anticipate future goals, consider consequences, and respond in an integrated or commonsense way.” (p. 135)

Best, Miller, and Jones (2009): “Executive function (EF) serves as an umbrella term to encompass the goal-oriented control functions of the PFC [prefrontal cortex].” (p. 180)

Borkowski and Burke (1996): “EF coordinates two levels of cognition by monitoring and controlling the use of the knowledge and strategies in concordance with the metacognitive level.” (p. 241)

Burgess (1997): “a range of poorly defined processes which are putatively involved in activities such as “problem-solving,” … “planning” … ‘initiation’ of activity, ‘cognitive estimation,’ and ‘prospective memory.’” (p. 81)

Corbett et al. (2009) “Executive function (EF) is an overarching term that refers to mental control processes that enable physical, cognitive, and emotional self-control.” (p. 210)

Crone (2009): “For example, during childhood and adolescence, children gain increasing capacity for inhibition and mental fl exibility, as is evident from, for example, improvements in the ability to switch back and forth between multiple tasks.” (p. 826)

Dawson and Guare (2010): “Executive skills allow us to organize our behavior over time and override immediate demands in favor of longer term goals.” (p. 1)

Delis (2012): “Executive functions reflect the ability to manage and regulate one’s behavior in order to achieve desired goals.” (p. 14)

Delis (2012): “Neither a single ability nor a comprehensive definition fully captures the conceptual scope of executive functions; rather, executive functioning is the sum product of a collection of higher level skills that converge to enable an individual to adapt and thrive in complex psychosocial environments.” (p. 14)

Denckla (1996): “EF has become a useful shorthand phrase for a set of domain-general control processes….” (p. 263)

Friedman, Haberstick, Willcutt, Miywake, Young, et al. (2007): “… a family of cognitive control processes that operate on lower-level processes to regulate and shape behavior.” (p. 893)

Funahashi (2001): “Executive function is considered to be a product of the coordinated operation of various processes to accomplish a particular goal in a flexible manner.” (p. 1)

Fuster (1997): 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)

Gioia, Isquith, Guy, and Kenworthy (2000): “The executive functions are a collection of processes that are responsible for guiding, directing, and managing cognitive, emotional, and behavioral functions, particularly during active, novel problem solving.” (p. 1)

Gioia and Isquith (2004): “The executive functions serve as an integrative directive system exerting regulatory control over the basic, domain-specific neuropsychological functions (e.g., language, visuospatial functions, memory, emotional experience, motor skills) in the service of reaching an intended goal.” (p. 139)

Hughes (2009): “The term executive function’ (EF), therefore, refers to a complex cognitive construct encompassing the whole set of processes underlying these controlled goal-directed responses to novel or difficult situations, processes which are generally associated with the prefrontal cortex (PFC).” (p. 313)

Lezak (1995): “Executive functioning asks how and whether a person goes about doing something.” (p. 42)

Lezak (1995): “Executive functions refer to a collection of interrelated cognitive and behavioral skills that are responsible for purposeful, goal-directed activity, and include the highest level of human functioning, such as intellect, thought, self-control, and social interaction.” (p. 42)

Luria ( 1966 ): “…Syntheses underlying own actions, without which goal-directed, selective behavior is impossible.” (p. 224)

Luria ( 1966 ): “…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)

McCloskey (2011): “It is helpful to think of executive functions as a set of independent but coordinated processes rather than a single trait.” (p. 2)

McCloskey (2006): “Executive Functions can be thought of as a diverse group of highly specific cognitive processes collected together to direct cognition, emotion, and motor activity, including mental functions associated with the ability to engage in purposeful, organized, strategic, self regulated, goal directed behavior.” (p. 1)

Miller and Cohen (2001): [our theory] “suggests that executive control involves the active maintenance of a particular type of information: The goals and rules of a task.” (p. 185)

Oosterlaan, Scheres, and Sergeant (2005): “EF encompasses meta-cognitive processes that enable efficient planning, execution, verification, and regulation of goal directed behavior.” (p. 69)

Pribram (1973): “… 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.” (p. 301)

Robbins (1996): “Executive function is required when effective new plans of action must be formulated, and appropriate sequences of responses must be selected and scheduled.” (p. 1463)

Roberts and Pennington (1996): EF “refers to a collection of related but somewhat distinct abilities such as planning, set maintenance, impulse control, working memory, and attentional control.” (p. 105)

Stuss and Benson (1986): “ Executive functions is a generic term that refers to a variety of different capacities that enable purposeful, goal-directed behavior, including behavioral regulation, working memory, planning and organizational skills, and self-monitoring.” (p. 272)

Vriezen and Pigott (2002): “Executive function has been defined in a variety of ways but is generally viewed as a multidimensional construct encapsulating higher-order cognitive processes that control and regulate a variety of cognitive, emotional and behavioral functions.” (p. 296)

Welsh and Pennington (1988): “Executive function is defined as the ability to maintain an appropriate problem-solving set for attainment of a future goal.” (p. 201)

A Brief Review of EF Models

Conceptualizations of EF have been largely driven by observations of individuals having suffered frontal lobe damage. Groups of such individuals were first described by Luria and reported to exhibit disorganized actions and strategies for everyday tasks. Initially this came to be referred to as dysexecutive syndrome. Such individuals tended to perform normally when clinical- or laboratory-based tests were used to assess more fundamental cognitive processes such as memory, learning, language, and reasoning, It was therefore determined that there must be some overarching system responsible for coordinating these other cognitive resources that appeared to be working inefficiently in patients with frontal lobe injuries. Recent functional neuroimaging studies have supported the theory of the PFC as responsible for EF, demonstrating that two parts of the prefrontal cortex, the ACC and DLPFC, appear to be particularly important for completing tasks thought to be sensitive to EF. In this section we will provide a brief chronological overview of the theories that appear to have driven our appreciation, definition, and understanding of EF.

Automatic and Controlled Processes

Donald Broadbent’s (1953) model of automatic and controlled processes, otherwise referred to as the filter model, proposed that a fi lter serves as a buffer that selects information for conscious awareness (Broadbent, 1958 ). When discussing competing stimuli, the filter determines which information must be distinguished as relevant or irrelevant (Barkley 2011a ). In other words, select information will pass through the filter (as relevant), while the remaining information is ignored (irrelevant) (Broadbent, 1958 ). Under this model, terminologies such as “sensory store” and “sensory filter” are utilized to explain the instrument in which processing of stimuli occurs at the preattentive level, focusing on properties such as the sex of the speaker or type of sound (Driver, 2001 ). Through a visual diagram, the processing of information could be represented with parallel lines up to a point in which processing is then managed with the filter (Schiffrin & Schneider, 1977 ), resembling a bottleneck, an additional name for Broadbent’s model (bottleneck theory) (Driver, 2001 ). If not for this fi lter/buffer, Broadbent believed that the system would become inundated or overloaded with information (Broadbent, 1958 ; Driver, 2001 ).

Cognitive Control

Posner and Snyder ( 1975 ) expanded upon the work of Broadbent and previous researchers with his “cognitive control” model (Posner & Snyder, 1975 ). This conceptualization utilized the bottleneck theory postulated by Broadbent by furthering the examination of the role of attention during specific higher-level tasks, including visual searches, for example (Posner & Snyder, 1975 ). However, Posner also suggested that cognitive control is needed to manage thoughts and emotions (Rueda, Posner, & Rothbart, 2004 ). By cognitive control, Posner refers to processes that guide behaviors, analogous to working definitions of executive functioning today. According to Posner & Snyder ( 1975 ) cognitive control was regarded as responsible in overwriting automatic responses, illustrating the selective nature of the model as well as the inhibitory nature (Posner and Snyder 1975 ). In this model, cognitive control allows one to adapt from situation to situation depending upon the goals of the individual (Checa, Rodriguez-Bailon, & Rueda, 2008 ).

Controlled Processes

Schiffrin and Schneider ( 1977 ) proposed that because our ability to attend is limited, certain stimuli must be favored over opposing stimuli. They studied the strength of a controlled processes theory of detection, search, and attention by comparing automatic detection with controlled search and concluded that by learning categories, controlled search performance also improved (Schiffrin & Schneider, 1977 ). In this dual processing theory, automatic processing activates a learned sequence of elements and proceeds automatically, while controlled processing entails a temporary activation of a sequence of elements that can be established rapidly, but they do require attention, nonetheless (Schiffrin & Schneider, 1977 ). Automatic processes are “effortless, rapid, unavailable to consciousness, and unavoidable; permanent connections that are developed with practice or training” (Schiffrin & Schneider, 1977 , p. 2). Without a need for active attention or active control, an individual is thus engaged in an automatic process. Controlled processes are “slow, effortful, and completely conscious; a temporary sequence of nodes activated under control of, and through attention by the subject” (Schiffrin & Schneider, 1977 , p. 2). With repeated practice, skills that were controlled can become automated, meaning that skills will not require as much attention resources to be completed (Schneider & Chein, 2003).

Supervisory Attentional System

Shallice ( 2002 ) constructed a model of the executive system called the contention scheduling/ supervisory attentional system model. Contention scheduling refers to the controlling mediator of inhibition of competing actions when selecting an action to be performed. The supervisory attentional system is a mediator for nonroutine situations in which inhibition may be necessary to make a decision during a novel encounter (Shallice, 1988 , 2002 ). When deficits exist in this supervisory attentional system, Shallice argues that executive disorders are possible (e.g., disinhibition) (Shallice, 2002 ).

Central Executive

Baddeley, Sala, and Robbins’s ( 1996 ) central executive hypothesis views the executive as a unified system with multiple functions, a homunculus of sorts. The central executive oversees the phonological loop, visuospatial sketchpad, and an episodic buffer. Below the central executive, Baddeley envisioned and described the following functions: time-sharing, selective attention, temporary activation of long-term memory, and switching of retrieval plans (Baddeley, 1986 ).

Cross Temporal Model

Fuster’s 1997 model of cross-temporal synthesis is based on three concepts: interference control, planning, and working memory. The theory proposed that the main goal of executive functions lie within organizing behavior (Barkley, 2011a ). Contrasting from previous models, especially Baddeley’s central executive model, Fuster does not “place a ghost in the machine” (Barkley, 2011a , p. 12). There is no central executive or single component within Fuster’s theory; rather, temporal mediation captures the interaction between short-term memory and the attention set (Fuster, 2000 ). In Fuster’s terminology, “new or recently learned behavior, sensory impulses are processed along the sensory hierarchy and into the motor hierarchy. Sensory information is thus translated into action, processed down the motor hierarchy to produce changes in the environment.”

Integrative Model

Miller and Cohen’s (2001) model focused on cognitive control and particularly the activities that represent maintenance of goals. They also refer to executive functioning as an umbrella term of cognitive processes under goal-directed behavior. In their model executive functioning is a top-down system serving to encourage sensory and motor processing areas into interacting with each other (Miller & Cohen, 2001). Maps are created between the inputs and outputs in this model, wherein bias signals guide activities along the neural pathways (Miller & Cohen, 2001).

Cascade of Control

Banich ( 2009 ) proposed that sequential cascade of brain areas attributed to maintaining attentional sets. According to Banich ( 2009 ) the DLPFC is the first to act using top-down attention to activate brain regions involved, and other regions of the cortex determine what information is necessary for an appropriate response. Finally, the posterior dorsal cingulate may serve as a catch all for the problems associated with selection thus far in this model (Banich, 2009 ).

Extended Phenotype

Barkley ( 2011a ) summarizes executive functioning with the term self-regulation composed of (1) working memory, (2) management of emotions, (3) problem solving, and (4) analysis and synthesis into new behavioral goals. Processes include working memory, planning, problem solving, self-monitoring, interference control, and self-motivation (Barkley, 2011b ).

A Developmental Perspective of EF

An important foundation for understanding the development of EF can be found in the works of Luria ( 1963 , 1966 , 1973 ). Luria’s neurodevelopmental model postulated specific developmental stages related to stages of higher cortical maturation. Luria suggested that various stages of mental development encountered as children mature provide a unique opportunity to study how EFs develop (Horton, 1987).

Luria ( 1966 ) postulated a number of stages by which neuropsychological functions critical for intelligence and EF are developed. These stages were thought to interact with environmental stimuli based on Vygotsky’s cultural and historical theory (Van der Veer and Valsiner, 1994). Vygotsky developed a complex theory related to language and thought processes. He postulated that environmental and/or cultural influences were important in understanding the development of neurological structures responsible for higher-level mental abilities, such as abstraction, memory, and attention. Luria expanded Vygotsky’s original theories (Vygotsky, 1997a, 1997b, 1997c, 1997d).

In 1966, Luria postulated that higher cortical functions involving EF required interaction of normal neurological development and specific environmental stimuli of a cultural, historical, and social nature of development. In this way, Luria’s thoughts are very consistent with current theory suggesting that particular phenotypes are shaped by environmental experience, leading to multi-finality or multiple endophenotypes. Thus, the result of the optimal interaction of neurological development and environmental stimuli would result in more efficient cortical functioning related to abilities such as language, attention, memory, intelligence, and EF.

In 1980, Luria proposed fi ve stages of human development:

Stage One: This stage begins in the first year of life and involves development of the brain stem structures such as the reticular activating system.

Stage Two: This stage involves the activation of the primary sensory areas for vision, hearing, and tactile perception and the primary motor areas of gross motor movement during the second year of life. This is consistent with Piaget’s stage of sensorimotor operations.

Stage Three: This stage involves development of single modalities in the secondary association areas of the brain as children enter their preschool years. The child’s mind recognized and reproduces various symbolic materials and develops the ability to model physical movement. This stage is consistent with Piaget’s concept of preoperational functioning.

Stage Four: This stage begins as the child enters first or second grade (7–8 years of age) as the tertiary areas of the parietal lobes are activated. The tertiary parietal lobes, the temporal parietal and occipital lobes join anatomically and involve coordination of the three major sensory input channels. During this stage, the child’s mind begins to make sense of sensory input and environmental stimulation. It is particularly important for the development of complex mental abilities. This stage fi ts Piaget’s concept of concrete operations.

Stage Five: During this stage, the brain becomes activated beginning at approximately 8 years of age, through adolescence and adulthood. This operation involves the frontal lobes; the area anterior to the central sulcus is crucial to the development of complex mental abilities involving abstract thinking, intentional memory, as well as the execution monitoring and evaluating for complex learning (Stuss & Benson, 1984). This stage fits Piaget’s concept of formal operations.

Beyond Luria’s stage theory of brain development, his theoretical account of dynamic brain function is perhaps one of the most complete of all theorists (Lewandowski, Lovett, Gordon, & Codding, 2008 ). Luria conceptualized four interconnected levels of brain-behavior relationships and neurocognitive functioning including (1) the structure of the brain, (2) the functional organization based on structure, (3) syndromes and impairments arising in brain disorders, and (4) clinical methods of assessment (Korkman, 1999). Luria’s theoretical formulations, methods, and ideas are well articulated in his books, Higher Cortical Functions in Man ( 1966 , 1980 ) and The Working Brain ( 1973 ). Luria viewed the brain as a functional mosaic, the parts of which interact in different combinations to subserve different cognitive processes (Luria, 1973 ). No single area of the brain functions without input from other areas; thus, integration is a key principle of brain function within a learning framework. Thought, problem solving, EF, and intelligent behavior result from interaction of complex brain activity across various areas. Luria’s ( 1966 , 1973 , 1980 ) research on the functional aspects of brain structures forms the basis for the development of the planning, attention, simultaneous, and successive processes (PASS) theory, described by Das, Naglieri, and Kirby ( 1994 ) and operationalized by Naglieri, Das, and Goldstein ( 2013 ).

In the Lurian framework of intellectual function, attention, language, sensory, perception, motor, visuospatial facilities, learning, and memory are complex, interrelated capacities. They are composed of flexible and interactive subcomponents, mediated by an equally flexible interaction neural network (Luria, 1962, 1980 ). These cognitive functions as conceptualized by Luria are modulated by three separate but connected functional units that provide the four basic psychological processes. These three brain “systems” are referred to as functional units because their neuropsychological mechanisms work in separate but interrelated systems. Multiple brain systems mediate complex cognitive functions. For example, multiple brain regions interact to mediate attentional processes (Mirsky, 1996; Castellanos et al., 2003 ). The executive functions managed by the third functional unit, as described by Luria, regulate the attentional processes of the first functional unit in sustaining the appropriate level of arousal and vigilance necessary for the detection of selection of relevant details from the environment. Consider the example of response inhibition. Inhibitory behavior allows a child to resist or inhibit responding to saline by irrelevant details during a task. This improves task performance. Response inhibition allows the child to focus over time on task-relevant features.

Prefrontal areas of the frontal lobes of the brain are associated with the third functional unit (Luria, 1980 ). The prefrontal cortex is well connected with every distinct functional unit of the brain (Goldberg, 2009). This unit is most likely responsible for planning and is involved with most behaviors we typically consider associated with executive function and executive function capacity (McCloskey, Perkins, and Van Divner, 2009 ). The third functional unit is also further differentiated into three zones with the primary zone in the motor strip of frontal lobe being concerned with motor output. The secondary zone is responsible for the sequencing of motor activity and speech production, whereas the tertiary zone is primarily involved with behaviors typically described as executive function. Damage to any of several areas of the frontal regions has been related to difficulties with impulse control, learning from mistakes, delay of gratification, and efficient attention. Because the third functional unit has rich connections with other parts of the brain, cortical and subcortical, there are often forward and backward influences to and from other regions such as the thalamic and hypothalamic and limbic areas. This set of connections, consistent with evolutionary theory, is reflecting a building of the brain over billions of years from a brain stem forward to the frontal lobes. Additionally a growing body of evidence points to a network of connected regions in the adjacent frontal and parietal lobes which have been implicated in higher auto-processing such as attention, decision making, and intelligent behavior (Kolb and Whishaw, 2009 ).

Luria wrote that the frontal lobe synthesized the information about the outside world and is the means whereby the behavior of the organism is regulated in conformity with the effect produced by its actions (Luria, 1980 , p. 263). The frontal lobes provide for the programming, regulation, and evaluation of behavior and enable the child to ask questions, develop strategies, and self-monitor (Luria, 1973 ). Other responsibilities of the third functional unit include the regulation of voluntary activity, conscious impulse control, and various linguistic skills such as spontaneous conversation. The third functional unit provides the most complex aspects of human behavior, including personality and consciousness (Das, 1980). A reciprocal relationship exists between the first and third functional units. The higher cortical systems both regulate and work in collaboration with the first functional unit while also receiving and processing information from the external world and determining an individual’s dynamic activity (Luria, 1973 ). This unit is also influenced by the regulatory effects of the cortex. Ascending and descending systems of the reticular formation enable this relationship by transmitting impulses from lower parts of the brain to the cortex and vice versa. Thus, damage to the prefrontal area can alter this reciprocal relationship so that the brain may not be sufficiently aroused for complex behaviors requiring sustained attention. In 2009, Goldberg described a breakdown in any portion of this complex, loop-like interaction between the prefrontal ventral brain stem and posterior cortex as producing systems of attention deficit. Castellanos et al. ( 2001 ) further hypothesize that the right prefrontal cortex and organs at the basal ganglia such as the substantia nigra and the cerebellum form a critical set of connections he described as “brain’s braking system.” These interconnections innervate and come online when inhibition, attention, and self-regulation are required.

The connection between units also links the psychological processes that are routed in each of the functional units. For PASS theory this means that the psychological processes of attention and planning are necessarily strongly related because planning often has conscious control of attention. In other words, one’s limited attentional resources are dictated by the plan for one’s behavior. The combination of attention and planning offer a functional description of executive function. However, attention and other PASS processes are influenced by many variables beyond planning. One of the influences is the environment. Novel encounters within daily life demand that individuals act in one way or another. The interaction of knowledge and several PASS processes are involved as individuals make judgments about similarities and differences between past situations and present demands, while estimating possible outcomes of action, even as acting. Humans are uniquely the only species capable of simultaneously thinking, evaluating, and acting. As Bromhill ( 2004 ) notes humans are able to think one thing while saying and or doing something else.

Luria’s organization of the brain into functional units was not an attempt to map out the precise locations with specific areas of higher cognition taking place. In fact, Luria believed no part of the brain works by itself; thus, no cognitive task solely requires simultaneous, successive planning or attention processing, or any other processes, but rather it is a matter of emphasis. Luria stated “perception of memorizing gnosis and praxis, speech and thinking, writing, reading and arithmetic cannot be regarded as isolated or even indivisible faculties” (Luria, 1973 , p. 29). Thus, an attempt to identify a fixed cortical location for any complex behavior would be considered a mistaken endeavor. Instead the brain should be conceptualized as a functioning whole composed of units that provide purpose.

Conclusion

Over the last 150 years, significant and critical advancements have been made in our understanding of the manner in which the brain regulates, manages, organizes, and helps organisms interface with their environment. It has now been well documented that to function effectively the brain requires an executive system. This EF system controls and manages other systems, abilities, and processes. Prefrontal areas of the frontal lobes primarily carry out this operation. These are parts of the brain that from an evolutionary perspective are more recently evolved. Thus, it is not surprising that human beings possess a complex EF system. Future research will continue to define, understand, and develop strategic and clinical strategies and interventions to facilitate the development and operation of the EF system.

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