The term "executive function" broadly refers to the voluntary regulation of subsidiary cognitive functions, including memory, language, and movements that require skill. Executive function includes filtering attention, goal-directed behavior, anticipation of the consequences of one's actions, and mental flexibility. The analysis of executive functions represents one of the most important research areas in contemporary neuroscience. Historically, executive function has been associated mainly with frontal lobe activity. Executive disorders have been documented in a diversity of conditions. Executive deficits early in dementia predict other behavioral disturbances, functional decline, and mortality. In elders, intellectual tasks and everyday activity programs may benefit executive function abilities.
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• The term “executive functions” includes problem solving, planning, inhibiting responses, strategy development and implementation, cognitive control, and working memory.
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• Frontal processes, although not synonymous with executive functioning, are integral to its function.
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• Three major variants of the executive dysfunction syndrome can be distinguished: orbitofrontal, medial, and dorsolateral.
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• Executive dysfunction has been documented in a diversity of conditions, including dementia, traumatic brain injury, white matter lesions, borderline personality disorder, substance abuse, multiple system atrophy, multiple sclerosis, schizophrenia, autism, attention deficit hyperactivity disorder, progressive supranuclear palsy, CADASIL, and Korsakoff syndrome.
Historical note and terminology
The term "executive function" is a relatively recent addition to behavioral neuroscience. The frontal lobes were observed to be selectively involved in the self-regulation of voluntary activities, such as problem solving, planning, inhibiting responses, strategy development and implementation, and working memory. Oppenheim, in the 1890s, associated personality changes with the orbital and mesial frontal lobes (133; 134). Luria distinguished 3 functional units in the brain: (1) arousal-motivation (limbic and reticular systems); (2) receiving, processing, and storing information (postrolandic cortical areas); and (3) programming, controlling, and verifying activity (frontal lobes) (110). Luria mentioned that this third unit has an executive role. "Executive function" as a term has been addressed by many but was coalesced by Lezak to discriminate cognitive functions from the "how" or "whether" of human behaviors (102). Lezak emphasized the fluidity of executive function and how the other cognitive and emotional functions depended on the hypothetical "executive." Baddeley grouped these behaviors into cognitive domains that included problems in planning, organizing behaviors, disinhibition, perseveration, reduced fluency, and initiation (11). Baddeley coined the term "dysexecutive syndrome." Each component of executive function has added to the array of cognitive processes, which include maintaining a problem-solving set for goal-directed behavior, interference control, flexibility, strategic planning, and the ability to anticipate and engage in goal-directed activity (45).
The definition of executive function encompasses actions fueled by conceptualizations, such as the ability to filter interference, engage in goal-directed behaviors, anticipate the consequences of one's actions, and the adaptive concept of mental flexibility (109; 110; 46; 73; 171). The concept of morality, ethical behaviors, self-awareness, and the idea of the frontal lobes as manager and programmer of the human psyche are also included.
Elliott defines executive function as complex processing requiring coordinating several subprocesses for particular goal (55). Intact frontal processes, although not synonymous with executive function, are integral to executive function. Although attempts to localize executive function to discrete frontal areas have been inconclusive, the emerging view is that executive function is mediated by dynamic and flexible networks. Neuroimaging results have implicated posterior, cortical, and subcortical regions in executive functioning (148; 141).
Phineas Gage became a metaphor for frontal lobe dysfunction, and the dysexecutive syndrome became synonymous with frontal lobe pathology. Harlow described Phineas Gage as a responsible foreman for a railroad company who suffered a tragic accident in which a tamping rod explosively rammed through his frontal lobes when he was supervising the construction of the Rutland and Burlington Railroad across Vermont (85). He was reported to have died in a drunken brawl. Initial analyses of Phineas Gage’s skull findings suggested that the injury involved both frontal lobes (42). Computer-generated 3-D reconstructions of a thin-slice computed tomography scan of the trajectory of the rod showed that the brain damage was limited to the left frontal lobe; furthermore, the ventricles and the vital intracranial vascular structures were not affected (146). It was of interest to Harlow that other cognitive functions (ie, memory, language, and sensory motor functions) remained intact, whereas personality was so greatly altered. In this instructional historical case, the “central executive” that regulates other neurologic functions was disrupted but not the basic other, foregoing neurologic functions
During the late 19th and early 20th centuries, clinical investigations documented diverse behavioral disorders in frontal disease. In 1880, Herman Oppenheim coined the term Witzelsucht, which was demonstrated by childishness and joking with "alleged" cheerfulness (133; 134). The term moria (reflecting "stupidity" and jocular attitude) was part of the change they observed. Oppenheim’s patients all had tumors involving right frontal areas, frequently invading the mesial and basal areas. Jastrowitz noted unconcern and "inappropriate cheerfulness" (94). "Frontal lobe syndrome" was conceptualized by Feuchtwanger (61). He correlated frontal pathology with behaviors that were not related to overt speech, memory, or sensorimotor deficits. He emphasized the personality changes in motivation, affective dysregulation, and the capacity to regulate and integrate other behaviors. Kurt Goldstein expanded the capacity of frontal lobe behaviors to include "the abstract attitude," initiation, mental flexibility, and gaining both the components and the gestalt of the complex environmental arena (77). Goldstein was also sensitive to the compensatory reactions of brain injured individuals coupled with premorbid personality characteristics.
Initially, it was not apparent that "frontal lobe" and "prefrontal cortex" were not synonymous with loci associated with executive dysfunction. The first 3 decades of the 20th century described precisely the structure of the prefrontal regions. Vincent was one of the first researchers to become aware that the connections to the prefrontal cortex were important to function (183). Early studies elucidated hypothalamic prefrontal connections (184), thus, beginning the research into autonomic and emotional responses of the mesial-orbital prefrontal cortex. It was here that bilateral mesial prefrontal damage that involved the supplementary motor area and the singular cortex was found to produce amotivational akinetic apraxia and motor planning deficits.
When World War II yielded focal brain deficits, frontal lobe pathology was extensively evaluated. Luria related prefrontal lobe activity with programming movement, inhibiting immediate responses as needed, abstracting, problem solving, verbal regulation of behavior, reorienting behavior according to the behavioral consequences, temporal integration of behavior, personality integrity, and consciousness (109; 110).
Currently, executive function research uses functional brain imaging techniques to pool collateral findings, look at antecedents, and use a large sample size to eliminate spurious variables; thus, brain regions that contribute to dysexecutive syndromes may prove to be more multifunctional (107). Functional imaging has demonstrated that adults and children with focal, especially frontal, right-hemispheric lesions display similar behaviors such as attentional deficits, inability to inhibit a response, and impersistence of activity (62).
The relationship between executive functions and so-called general intelligence is not well-defined yet (66). Barbey and colleagues evaluated impairments on the Wechsler Adult Intelligence Scale and Delis-Kaplan Executive Function System in 182 patients with focal brain damage in relation to voxel-based lesion-symptom mapping (14). Abnormal performance in these tests was observed following damage to a distributed network of left lateralized brain areas (frontal and parietal cortex and white matter association tracts). It has also been pointed out that some executive function tests, such as the Wisconsin Card Sorting Test and Verbal Fluency, are closely linked to fluid intelligence. Departing from the hypothesis that fluid intelligence is related to executive function, a confirmatory factor analysis on a mixed neuropsychiatric and nonclinical sample found a high correlation between fluid intelligence and executive functioning (0.91), with working memory being the most profound indicator (180). A moderate to high correlation between crystallized intelligence and executive function was also present. The authors concluded that this study clearly supported the strong association between executive function and intelligence, particularly fluid intelligence. Executive dysfunction observed in some clinical conditions such as Parkinson disease can be interpreted to reflect a decrease in fluid intelligence (149).