Executive Function The Search for an Integrated Account

In general, executive function can be thought of as the set of abilities required to effortfully guide behavior toward a goal, especially in nonroutine situations. Psychologists are interested in expanding the understanding of executive function because it is thought to be a key process in intelligent behavior, it is compromised in a variety of psychiatric and neurological disorders, it varies across the life span, and it affects performance in complicated environments, such as the cockpits of advanced aircraft. This article provides a brief introduction to the concept of executive function and discusses how it is assessed and the conditions under which it is compromised. A short overview of the diverse theoretical viewpoints regarding its psychological and biological underpinnings is also provided. The article concludes with a consideration of how a multilevel approach may provide a more integrated account of executive function than has been previously available. KEYWORDS—executive function; frontal lobe; prefrontal cortex; inhibition; task switching; working memory; attention; top-down control Like other psychological constructs, such as memory, executive function is multidimensional. As such, there exists a variety of models that provide varying viewpoints as to its basic component processes. Nonetheless, common across most of them is the idea that executive function is a process used to effortfully guide behavior toward a goal, especially in nonroutine situations. Various functions or abilities are thought to fall under the rubric of executive function. These include prioritizing and sequencing behavior, inhibiting familiar or stereotyped behaviors, creating and maintaining an idea of what task or information is most relevant for current purposes (often referred to as an attentional or mental set), providing resistance to information that is distracting or task irrelevant, switching between task goals, utilizing relevant information in support of decision making, categorizing or otherwise abstracting common elements across items, and handling novel information or situations. As can be seen from this list, the functions that fall under the category of executive function are indeed wide ranging. ASSESSING EXECUTIVE FUNCTION The very nature of executive function makes it difficult to measure in the clinic or the laboratory; it involves an individual guiding his or her behavior, especially in novel, unstructured, and nonroutine situations that require some degree of judgment. In contrast, standard testing situations are structured—participants are explicitly told what the task is, given rules for performing the task, and provided with information on task constraints (e.g., time limits). Since executive function covers a wide domain of skills, there is no single agreed-upon ‘‘gold standard’’ test of executive function. Rather, different tasks are typically used to assess its different facets. One classic test often used to assess the compromise of executive function after brain injury is the Wisconsin Card Sorting Test. This task is thought to measure a variety of executive subprocesses, including the ability to infer the categories that should guide behavior, the ability to create an attentional set based on those abstract categories, and the ability to switch one’s attentional set as task demands change. Briefly, individuals must deduce from the experimenter’s response the rule by which the cards should be sorted (rather than being told the rule explicitly; see Fig. 1a). After the initial rule is learned successfully, the examiner changes the rule without informing the individual. At this point the old rule must be rejected, the new rule discovered, and a switch made from the old rule to the new. The ability to exhibit such flexible readjustment of behavior is a cardinal characteristic of executive function. Individuals with frontal lobe damage and children younger than 4 years (who are typically tested on a two-dimensional version of the sorting task) tend to persist in sorting items according to the previous and now inappropriate rule. Address correspondence to Marie Banich, Director, Institute of Cognitive Science, University of Colorado at Boulder, UCB 0344, Boulder, CO 80305; e-mail: marie.banich@colorado.edu. CURRENT DIRECTIONS IN PSYCHOLOGICAL SCIENCE Volume 18—Number 2 89 Copyright r 2009 Association for Psychological Science Cognitive psychologists have attempted to disentangle the different executive subprocesses that underlie performance on the Wisconsin Card Sorting Test, as well as to identify other executive subprocesses. For example, the ability to switch mental sets has been studied by presenting individuals with multidimensional stimuli (e.g., a colored numeral) along with a cue that indicates the attribute on which a response should be based (e.g., color, or whether the number is odd or even). Individuals are slower to respond and make more errors on trials requiring a task switch (e.g., categorize by color preceded by categorize by odd/even) than they do on those that do not (e.g., categorize by color preceded by categorize by color), indicating that task switching requires executive control (Monsell, 2003). In other executive tasks, decisions must be based on taskrelevant information in the face of distracting information. One such measure of this ability is the Stroop task, in which a word’s color must be identified while ignoring the word itself. Since word reading is more automatic than color naming, executive control is required to override the tendency to read or to respond on the basis of the word rather than the ink color. The need for such control is reflected in slower responses when the word names a competing ink color (e.g., the word ‘‘red’’ printed in blue ink) than when it does not (e.g., the word ‘‘sum’’ in red ink or the word ‘‘red’’ in red ink). Other tasks, such as the Tower of London task, examine the ability to plan and sequence behavior towards a goal. In this task, a start state and a goal state are shown, and the individual must determine the shortest number of moves required to get the balls from the starting state to the goal state (see Fig. 1b). An inability to solve the problems, taking more steps than necessary, and/or impulsively starting to move the balls before planning are all symptoms of executive dysfunction on this task. THE COMPROMISE OF EXECUTIVE FUNCTION Psychologists are interested in executive function because it is critical for self-directed behavior, so much so that the greater the decrement in executive function after brain damage, the poorer the ability to live independently (Hanks, Rapport, Millis, & Deshpande, 1999). Normal children, adolescents, and older adults also show decrements in executive function. Most notable in children is their perseveration when required to switch tasks. Although they can correctly answer questions about what they should do, they nonetheless are often unable to produce the correct motor response (Zelazo, Fyre, & Rapus, 1996). Similarly, parents often wonder why teenagers take risks and make imprudent decisions even though they seem to ‘‘know’’ better. This demonstrated knowledge about abstract rules coupled with an inability to implement them, especially in the face of distracting or conflicting information, is reminiscent of that observed in children. The ability to plan ahead in multistep processes, to learn about contingences between reward and punishment in multifaceted decision-making tasks, and to exert inhibitory control and reduce impulsive behavior continues to increase during the teenage years and, in fact, well into the early 20s (Steinberg, 2007). Executive function is also the cognitive ability most affected by aging (e.g., Treitz, Heyder, & Daum, 2007), with even more severe decline associated with mild cognitive impairment and Alzheimer’s disease. Finally, executive function is compromised across a large number of psychiatric illnesses, including schizophrenia, bipolar disorder, ?