9.4 Executive Functions

In recent years, cognitive psychologists have seen working memory not as an isolated process but as part of what are termed executive functions—relatively basic and general-purpose information-processing mechanisms that, together, are important in planning, regulating behavior, and performing complex cognitive tasks (Miyake & Friedman, 2012; Miyake et al., 2000). Most researchers agree that executive functions consist of three related components: (a) working memory, or updating, monitoring, and rapidly adding/deleting the contents of working memory; (b) switching, shifting flexibly between different tasks or mind-sets; and (c) inhibition, preventing a cognitive or behavioral response, or keeping unwanted information out of mind. Individual differences in executive functions are related to performance on other cognitive tasks, such as IQ, reasoning, and school grades (Friedman et al., 2006; Richland & Burchinal, 2013), as well as important socioemotional phenomena (see Chapter 13, p. 518). The various components of executive function have been assessed by a variety of tasks.

An example of a switching task is the Wisconsin Card Sorting Test (WCST), in which subjects are given sets of cards with different objects on them (such as squares, stars, and circles) that vary in color and number (see Figure 9.8). Subjects are asked to sort the cards into specific categories (for example, according to color, number, or shape), which is reinforced by the examiner. After several trials and without notice, the examiner switches reinforcement to another category. For instance, the initial category may be color, in which case subjects would be reinforced for sorting all the green cards in one group, the red ones an another, and so on, regardless of the number or shape of the items on the cards. The examiner may then switch from color to number, so that all target cards are now to be placed according to number (one, two, three, or four), with color and shape being irrelevant. Subjects are given feedback after a mistake, so they should presumably be able to learn a new classification scheme after only a few trials.

Figure 9.8: The Wisconsin Card Sorting Task Subjects start sorting cards by one dimension (color, for instance) then, without notice, they are reinforced for sorting by another dimension (shape, for instance). The number of errors on “switch” trials is a measure of the executive function of switching.

(With permission from E. A. Berg (1948). A simple objective technique for measuring flexibility in thinking. Journal of General Psychology 39:15-22. Copyright © 1948. Reproduced by permission of Taylor & Francis Ltd. http://www.tandf.co.uk/journals and Taylor & Francis LLC http://www.tandfonline.com)

Inhibition has also been assessed by a variety of relatively simple tasks. For example, the Stroop task discussed earlier in this chapter (see Figure 9.6) is used to assess inhibition. To what extent can people inhibit the dominant response (to say the color word “blue,” for example) and instead identify the color the word is written in (in this case, green)?

Updating, or working memory, is assessed by tasks like those described in the previous section looking at working-memory span, as well as some dual tasks, such as those assessing the effects of talking on the phone and driving.

Four General Conclusions About Executive Functions

Akira Miyake and Naomi Friedman (2012) have looked at over a decade of research and arrived at four general conclusions about executive functions. First, executive functions show both unity and diversity. This means that performance on the various types of executive functions (updating, switching, and inhibition) all correlate with one another. That is, people who perform well on updating tasks are likely to perform well on switching and inhibition tasks as well. This suggests that the various tasks are tapping some common underlying cognitive ability. However, these correlations are not perfect, indicating that each type of executive function is also assessing some unique abilities. In particular, Miyake and Friedman have determined that updating (working memory) and switching (the ability to shift between tasks) each contributes something beyond the “unity” factor common to all executive functions, whereas inhibition abilities do not add anything to the equation beyond the general, unity factor. This is expressed graphically in Figure 9.9.

Figure 9.9: Schematic representation of the unity and diversity of three executive functions (EFs) The various EFs are related to one another, forming a common EFs factor. However, updating (working memory) and switching (shifting) also contribute independently to performance beyond what can be accounted for by the common EFs factor.

(With permission from Miyake, A. & Friedman, N. P. (2012). The nature and organization of individual differences in executive function: Four general conclusions. Current Directions in Psychological Science, 21, 8-14. Copyright © Akira Miyake and Naomi P. Friedman, 2012. SAGE Publications.)

Second, there is a substantial genetic component to executive functions. For example, by looking at performance of different types of executive functions by people with different degrees of genetic relatedness (for instance, identical twins, biological siblings, and adopted siblings), one can get an estimate of the heritability of a trait, or the degree to which individual differences in a trait can be attributed to inheritance. Research by Friedman and her colleagues (2008) showed that the heritability of executive functions was quite high—higher than what is typically reported for IQ or personality.

This does not mean that executive functions cannot be altered by experience, however. When people experience different environments (including training environments), the level of a trait can vary even when heritability is high. A number of studies have trained people ranging in age from preschoolers to adults in various aspects of executive functions and have reported significant improvements (Best, 2011; Dahlin et al., 2008; Diamond, 2012; Diamond & Lee, 2011). In fact, physical exercise, especially combined with “character training” as in martial arts (Diamond, 2012), has been shown to relate to executive functions, with people who exercise more having better executive functions (Hillman et al., 2009); and training studies with older adults have demonstrated that increases in physical exercise are associated with corresponding increases in exexuctive finctions (Colcombe & Kramer, 2003). One unexpected environmental influence on executive functions is the number of languages a person speaks. Recent research has shown that bi- and multilingual people have better executive-function abilities than monolinguals (Bialystock, 2010).

Third, executive functions are related to and predictive of important clinical and societal outcomes. For example, externalizing behaviors (where one “acts out” such that one’s behavior adversely affects other people, including conduct disorder and opposition defiant disorder), attention-deficit/hyperactivity disorder (ADHD), excessive risk taking, and substance abuse are all related to low levels of behavioral inhibition and are associated with executive functions, such that people with better executive functions have fewer behavior problems (Young et al., 2009). More generally, executive functions are related to the ability to regulate one’s behavior and emotions (for example, to display self-discipline), beginning in early childhood (Kochanska, Murray, & Harlan, 2000), through adulthood (Pronk et al., 2011), and into old age (von Hippel & Dunlop, 2005).

Fourth, there is substantial developmental stability of executive-function abilities. Although all aspects of executive functions improve over childhood, children (and even infants) who perform well on executive-function tasks tend to develop into adults with high executive-function abilities.

Executive functions can be thought of as a set of low-level cognitive abilities that, in combination, make it possible for people to regulate their thoughts, emotions, and behavior. These abilities improve with age, decline in old age, and at all ages are associated with psychological functioning. Several researchers have speculated that the evolution of executive functions was an important component in the emergence of the modern human mind (Causey & Bjorklund, 2011; Geary, 2005a). The abilities to keep an increasing number of items in mind at one time, resist distractions, inhibit inappropriate behavior, and regulate one’s emotions and actions are critical to effective functioning in any social group, as well as for activities such as making tools, hunting, and preparing meals, among many others. These abilities are better developed in humans than in other primates and may be a key to understanding both human cognition and human evolution.

Neurological Basis of Executive Functions

Research has also found correlates of executive functions with brain function and structure, especially the prefrontal cortex, and we look briefly at some of this research next. There is no single area of the brain that is responsible for the various components of executive functions, but the prefrontal cortex has been identified as a critical area for the control of thought and behavior (Miller & Wallis, 2012). The prefrontal cortex appears to be the neural hub for executive functions (Huey et al., 2006). It is the part of the brain that somehow organizes the efforts of the other portions of the brain and keeps them focused on the task. The prefrontal cortex receives information from the sensory cortex and is connected to structures in the motor system, the limbic system (important in memory, motivation, and emotional expression), and the basal ganglia. It is thus well placed to play a major role in the control of behavior and cognition.

One of the earliest cases in the medical annals demonstrating the role of the prefrontal cortex in emotional regulation is that of Phineas Gage (Damasio et al., 1994). Gage, a railroad employee, was in the wrong place at the wrong time, when an explosion sent a metal bar through his cheek and out the top of his head (see Figure 9.10). The accident destroyed much of his prefrontal cortex, but, surprisingly, he survived and seemed not to have lost any of his intellectual abilities. However, in many other respects, Gage was a changed man. He was unable to plan the work he and his crew needed to accomplish and frequently spoke in a profane, rude, and irreverent manner, all counter to his preinjury personality. In essence, he was unable to control his impulses. Patients with prefrontal lobe damage, like Gage, often lack empathy, show alterations in mood and emotional expressions, have difficulty planning and making decisions, and generally have difficulty inhibiting thoughts and behaviors. For example, patients with frontal lobe damage perform poorly on the Wisconsin Card Sorting Tasks discussed earlier. When the rules change (sort by color, not by number), they are unable to make the switch, but rather continue to sort by the previous rule.

Figure 9.10: Phineas Gage This reconstruction of the injury suffered by Phineas Gage shows how the metal rod disconnected his frontal lobes from other parts of his brain. Although Gage’s intelligence was presumably unchanged after the accident, his personality and ability to plan and make decisions were radically affected.

Patrick Landmann/Science Source

ADHD is associated with delays in the development the frontal cortex. For instance, in one study, development of the frontal cortex of 7- to 13-year-old children with ADHD lagged about 3 years behind those of children without ADHD, whereas their motor areas developed slightly earlier (Shaw et al., 2007). This uneven pattern of brain development may account for the increased fidgeting and restlessness seen in children with ADHD.

More specific brain regions can also be identified that are associated with particular aspects of executive functions. For example, working-memory tasks involve the anterior portion of each prefrontal lobe. In neuroimaging studies, increased activity in the prefrontal cortex occurs whenever a person is deliberately holding either verbal or visual information in mind (Nee et al., 2008). In one study, neural activity in the prefrontal cortex was significantly greater on trials in which the person successfully kept the information in mind than in unsuccessful trials (Sakai et al., 2002).

Although the relation of the prefrontal cortex to the regulation of behavior has been known at least since the days of Phineas Gage, new neuroimaging techniques are permitting scientists to get a closer look at important brain/cognition relationships. Moreover, as we learn more about how the brain is involved in the control of our thoughts and behaviors, we’re bound to discover interesting individual differences. It’s very likely that different people take alternative neural routes to achieve similar goals (Braver et al., 2010).