Executive function is a term that describes a wide range of cognitive behaviors and processes. It is broad enough of a term that some people simply describe it as, “what the frontal lobes do.” When asked what exactly the frontal lobes do do, some revert to the circular definition of “executive functions.” However, executive functions are distinct from – but related to – what the frontal lobes do. The frontal lobes are involved in motor functions (e.g., pre-motor and primary motor areas), eye movement (e.g., frontal eye fields), memory (e.g., acetylcholine-producing portions of the basal forebrain), and language (BA 44,45 or Broca’s area). In addition, some executive functions incorporate areas of the brain outside the frontal lobes – the parietal lobes or basal ganglia, for example. Like many cognitive domains, executive functions are part of a distributed network of brain structures and regions.
Most neuropsychologists however, would define (or at least accept the following definition of) executive function similar to this: Executive function is the ability to selectively attend to, work with, and plan for specific information. This means that executive function is deciding what information, cognitions, or stimuli are relevant, holding and working with that information, and then planning what to do with it. As such, executive function is largely the roles of planning and organization. It is also the ability to recognize and learn patterns (i.e., cognitive sets) but also have the cognitive flexibility to respond to set changes and make a shift in set. Executive function also involves being able to select the appropriate response or behavior while at the same time inhibiting inappropriate responses or behaviors.
Executive functions have been compared to the conductor of an orchestra who, in order to make sense of the disparate instruments, sounds, and parts, must coordinate the members and lead the efforts of all the components of the orchestra. Executive functions also have been compared to chief executive officers of companies. These comparisons demonstrate that executive functions are arguably the most complex and highest of all cognitive functions. However, just like most other cognitive functions, executive functions are comprised of relatively simple processes (e.g., attention and processing speed) – it is just the unique combination of these more basic processes that makes executive functions so powerful.
One potential problem with executive function as a cognitive domain is that it is large and loose. Many tests have been developed, or at least used, to assess executive function (e.g., Wisconsin Cart Sort Test, Stroop Color-Word Task, clock drawing, and so forth). Even though all such tests are used as measures of executive functioning, scores on them do not always correlate highly with each other. They do not always cluster together when subjected to principal components analysis or even structural equations modeling. This means that even though neuropsychologists have many purported tests of executive function, they all seem to measure different aspects of executive function. This might partially result from executive functioning tests being differentially affected by basic cognitive processes such as processing speed.
Even though, as previously mentioned, I do not believe executive functions and frontal lobe functions are synonymous terms, are we able to localize executive functions to the frontal lobes? Largely we can. The most evidence from neuroimaging studies and neurological injuries demonstrate that the prefrontal cortex – the area of the brain that is phylogenetically youngest and most advanced and as such, proportionately larger in humans than any animal – is necessary (but not necessarily sufficient) for executive functioning. When this area is disrupted in humans, they exhibit poor decision-making skills, including poor planning and poor maintenance or self-regulation of behavior. One area of the prefrontal cortex particularly involved in executive functions is the dorsolateral prefrontal cortex (area 46) – although both the orbitofrontal and anterior cingulate are involved in aspects of executive functions.
In 1986 Alexander, Delong, and Strick published their seminal work on five parallel and closed cortico-striato-thalamo-cortical loops. These frontal-subcortical circuits were hypothesized to be involved in a range of behaviors and cognitions based on the varying cortical connections of the loops. Previously, many researchers did not well-understand the role that the basal ganglia played in any sort of “higher” function; in fact, most viewed the basal ganglia as involved mainly in motor behaviors. Alexander, Delong, and Strick’s article set off a flurry of research into the functions of these frontal-subcortical circuits, which have been verified as existent in humans (Middleton & Strick, 2000). Over time different theories have modified these circuits, including that they are composed of direct, indirect, and hyperdirect pathways, which all function at different speeds or timings to allow the basal ganglia to regulate behavior. Mink (1996) proposed that actions (e.g., producing a specific word) are regulated by the direct and indirect pathways, which serve as large components of our ability to select and inhibit correct and incorrect responses, respectively. It is as if each individual fronto-cortical loop allows us to properly attend to the correct behavior or response and inhibit all other behaviors or responses, much like the DLPFC and orbitofrontal cortex and their associated loops are involved in the selection and inhibition of behavior, both major aspects of executive function.
Just as damage to the dorsolateral prefrontal cortex (DLPFC) produces deficits in executive function, damage to any part of the DLPFC loop also results in executive dysfunction. Benke, Delazer, Bartha, and Auer (2003) presented two clinical cases of patients with left caudate lesions (the lesions also affected part of the anterior limb of the internal capsule as well as portions of the putamen and pallidum; however, the infarcts affected the caudate the most). Among other deficits, both patients had executive function impairments, including problem-solving deficits, many perseverative errors, and set-shifting problems. Even though the patients had no direct DLPFC damage, they exhibited similar deficits to patients with DLPFC lesions. These executive deficits persisted over time.
As a cognitive domain, and even as broad as it might be, executive functioning has ecological validity. Price and colleagues (2008) found that executive dysfunction was related to greater difficulty performing IADLs. Specifically, patients with executive dysfunction had more difficulty performing IADLs than patients with memory deficits did. Thus, how quickly, flexibly, and accurately people can organize, solve, plan, or attend to specific neuropsychological tasks seems to correlate with their accomplishment of everyday tasks of life, such as finances, driving, and shopping.
References
Alexander, G. E., DeLong, M. R., & Strick, P. L. (1986). Parallel organization of functionally segregated circuits linking basal ganglia and cortex. Annual Review of Neuroscience, 9, 357-381.
Benke, T., Delazer, M., Bartha, L., Auer, A. (2003). Basal ganglia lesions and the theory of fronto-subcortical loops: Neuropsychological findings in two patients with left caudate lesions. Neurocase, 9, 70-85.
Middleton FA, & Strick PL. (2001). Basal ganglia output and cognition: evidence from anatomical, behavioral, and clinical studies. Brain Cogn., 42, 183-200.
Mink, J. W. (1996). The basal ganglia: Focused selection and inhibition of competing motor programs. Prog Neurobiol, 50, 381-425.
Price, C.C., Garvan, C., and Monk, T. (2008). Type and severity of cognitive impairment in older adults after non-cardiac surgery. Anesthesiology, 108, 8-17.
One experiment about cognitive brain functioning is the word superiority effect findings of Dr. Reicher in 1969. In this experiment either a word or a non-word (string of letters) is flashed on a screen. The subject is asked if the stimulus contained one of two letters, say a “C” or an “E”. When the stimulus did not resemble a word (e.g., XXCX) subjects were correct in identifying the target letter about 80% of the time. When the string of letters was similar to a word but not one (e.g., FELV) the subjects also correctly identified the target letter 80% of the time. However, the interesting finding was that when the stimulus was a word (e.g., TEND), subjects were correct in identification 90% of the time. So the word superiority effect is that subjects are most accurate in identifying a target letter when it is contained in a word as opposed to a string of letters.