Components of Executive Function

Executive function has been conceptualized in many different ways, with several people presenting different views as to how it may operate. An homogenous definition of executive function has recently been criticised, with researchers arguing

that one construct does not sufficiently describe the complexity of impairments seen in children and adults with difficulties in executive function (Baddeley, 2002;

Kimberg, D'Esposito, & Farah, 1997). Some researchers view executive function as a functional construct, which draws on various processes involved in problem-solving, including the anticipation or formation of a goal, planning to reach that goal, carrying out the behaviours and the subsequent evaluation of performance (Lezak, 1983; Zelazo & Muller, 2002). While these problem-solving frameworks are useful in terms of helping to break down the different stages of analysis for which children may invoke executive strategies, they do not delineate the cognitive mechanisms by which effective executive function is achieved. Thus, although executive function may represent a theoretically useful, overarching latent construct, it is probably accomplished with the aid of a number of component skills (Kimberg et al., 1997).

This componential view of executive function is supported by research that shows that different developmental disorders are associated with deficits in different types of executive function. For example, children with ADHD usually demonstrate deficits on tasks purported to tap inhibitory control, while their performance on tasks that assess fluency or mental flexibility is usually similar to that of their peers. Children with autism, however, are more likely to struggle with tasks that require set- shifting and their deficits are generally more pronounced, suggesting different profiles of executive impairment in each of these two developmental disorders (Geurts, Verte, Oosterlaan, Roeyers, & Sargeant, 2004; Ozonoff & Jensen, 1999; Pennington & Ozonhofff, 1996).

Support for a multi-component view of executive function also comes from factor analytic studies. In these studies, researchers have administered batteries of tests to children of various ages (3-21 years) and then performed principal

function variables to determine whether they represent a common underlying construct as opposed to multiple factors. In general, these studies have tended to support the later argument, but have also acknowledged an underlying unity or common latent construct for executive function tasks, with correlations between tasks being small to moderate. Studies using PCA have variously supported separate dimensions of inhibitory control (Brocki & Bohlin, 1999; Levin et al., 1991; Welsh, Pennington, & Groisser, 1991), planning (Levin et al., 1991; Welsh et al., 1991), fluency and processing speed (Anderson, Anderson, Northam, Jacobs, & Catroppa, 2001; Brocki & Bohlin, 1999; Welsh et al., 1991) and working memory (Brocki & Bohlin, 1999).

However, it must be acknowledged that these studies rely on the researchers’ interpretations of what observed constructs may represent and tasks have varied considerably across studies. More recently, researchers have employed CFA to try to confirm their assessments of the latent constructs that executive tasks may be

tapping. These studies have shown support for a separate executive dimension of set- shifting (Huizinga, Dolan, & van der Molan, 2006; Pennington, 1998; van der Sluis et al., 2007). In addition, two of these studies have indicated working memory (the ability to maintain information on line during cognitive processing) to be a separate factor (Huizinga et al., 2006; Pennington, 1998), while a third study deemed this factor ‘updating’ (replacing previously relevant information with new information relevant to the task; van der Sluis et al. 2007). The evidence for an ‘inhibitory control’ factor in these studies has been less consistent. While one study isolated an inhibitory control factor (Pennington, 1998), the others found that tasks for which children were asked to inhibit one response in favour of another did not tend to reflect a common latent variable (Huizinga et al., 2006; van der Sluis et al., 2007).

by the neurological literature. Generally, there is a division of the frontal cortex into the ventro-medial orbitofrontal cortex, the dorsolateral prefrontal cortex and the anterior cingulate cortex. These three areas, though closely associated, also appear to have different responsibilities. The dorsolateral prefrontal cortex appears especially important for spatial planning, working memory and language, generally deemed ‘cold’, abstractive, cognitive aspects of executive function (Zelazo, Qu, & Muller, 2005). The ventromedial prefrontal cortex is more densely connected to subcortical limbic regions and emotional areas of the brain and appears to be more related to social judgement, delay of gratification and reward-based processing. These aspects of executive function are generally thought to be ‘hot’ elements, in that they involve an emotional element (Zelazo et al., 2005). Finally, the anterior cingulate cortex is related to drive, arousal and response selection. It has been especially implicated in tasks that require the resolution of response conflict and interacts with the motor systems (Banfield, Wyland, Macrae, Munte, & Heatherton, 2004; Fuster, 2003; Heyder, Suchan, & Daum, 2004).

Thus, it seems that executive function can be conceptualised as a cognitive system of different skills that act together to allow for effective goal-directed problem-solving (Stuss, Shallice, Alexander, & Picton, 1995). Theoretical

discussions of the various elements of executive function generally include working memory, attentional regulation (including inhibitory control, selective and sustained attention), set-shifting or cognitive flexibility and more meta-cognitive skills such as planning, initiation and self-monitoring (Anderson, 2002; Anderson et al., 2002; Banich, 2004; Denckla, 1996; Gioia, Isquith, & Guy, 2001; Hughes et al., 2004; Lezak, 1983; Miyake et al., 2000; Samango-Sprouse, 1999). Although fluency or abstraction is also commonly mentioned, this skill has been found to mature more

slowly than others and is therefore less easily assessed in young age groups such as that in the current study (Klenberg et al., 2001).

Tasks for this study were therefore specifically selected on the basis of these factor analysis studies and theoretical models of executive function, with a recognition that more complex tasks will often tax multiple areas of executive function and that the system probably operates in an integrative and interdependent manner (Anderson, 2002; Strauss, Sherman, & Spreen, 2006). The key executive functions considered in this study are briefly described below. To assist the reader in the interpretation of studies reviewed in this chapter, Table 2.1 provides an overview of tasks that are commonly used to assess executive function and the key domains that they are thought to assess.

Table 2.1: Outline of Measures Commonly Used to Assess Executive Function

Task Construct measured Description

A-not –B/ search tasks

Working memory/ inhibitory control

Preliminary trials encourage an infant to search for a reward in one location. Following several trials, the object is hidden in an alternative location within view of the infant. After a delay is imposed, young infants generally search in the previously rewarded location.

Card sorting tasks

E.g. Wisconsin Card Sorting Task (WCST) and Dimensional Change Card Sort

Set-shifting / mental flexibility

Inhibitory control Perseveration

The task involves sorting a set of cards according to different rules. In one condition, the cards are sorted according to one dimension, e.g. according to colour. In the other, they are sorted according to a different dimension (e.g. shape). For younger children, the rule is articulated, but participants must infer the rule according to reward feedback for the WCST.

Continuous performance tests and Go/no-go tests

Inhibitory control Sustained attention Processing speed

Pictures, numbers or words are presented at short intervals on a computer screen. The participant responds to each of these stimuli, except for a pre- selected stimulus that is presented infrequently. The participant must withhold a response to this stimulus. A participant’s incorrect responses and response times are recorded on the computer.

Stroop Test

E.g. Stroop Tasks Inhibitory control

The participant must respond to a stimulus such as the word ‘green’ with a non-prepotent response that generally requires reduced processing speed, e.g. the participant must name the colour of the ink the word ‘green’ is written in or say ‘night’ when viewing a picture of a sun.

Memory span tests

E.g. Digit span test, sentence span tests or counting span tests

Working memory

Some of these tests, e.g. Digit Span, require the participant to repeat back the cues in sequence or in reverse order. Others, such as the counting span test, involve remembering a set of digits or words, while simultaneously completing a reasoning task.

Fluency tasks

Verbal fluency Design fluency Mental abstraction

These tasks require the participant to generate novel information within some specified parameters. In a verbal fluency task, the participant must generate as many words as possible within a set (animals, for example) in a given time frame. In a design fluency task, the participant must draw patterns within specified grid lines without replicating a previous pattern.

Tower of Hanoi / Tower of London Tasks

Planning/ Problem solving

Coloured disks or beads are arranged on pegs. The participant must transfer disks to a pre-specified goal position. Rules must also be followed (e.g. a large disk cannot be placed on a smaller one), necessitating forethought and sequencing. Motor sequencing

Tasks

E.g. hand game, tapping tasks

Inhibitory control Motor fluency

The participant must replicate the actions of the researcher. The researcher then asks the participant to do the opposite of what they are doing, for e.g. In Luria’s hand game, the participant must make a fist when the researcher points a finger and vice versa.

Trail Making Tasks Set-shifting

During the first sequence of this task, the participant connects numbers/letters in sequential order. During the second phase, the participant must switch between numbers and letters (i.e. 1 - A - 2 - B).

2.1.1 Working memory

Working memory has been defined as a system of processing units that allow us to hold goal-related information ‘on line’ while we process or mentally operate on it (Fuster, 2003; Pennington, 1994; Welsh, 2002; Zoelch, Seitz, & Schumann-

Hengsteler, 2005). In accordance with these definitions, working memory is generally thought to be more complex than short term storage components of memory and is generally measured using tasks that require both the storage and the simultaneous utilisation or processing of information (Denckla, 1996).

Although various conceptualizations of the structure of working memory exist, the most widely recognized is that of Baddeley and Hitch (Baddeley, 1986, 1998, 2002), who make the distinction between three key systems: 1) the phonological loop, which retains verbal information for short periods of time, 2) the visual-spatial sketchpad, which operates as a mental drawing board, allowing for the retention of spatial information, and 3) an executive control system that operates on these storage systems, negotiating competing responses and sequencing mental plans. Evidence for the existence of these three separate systems comes from experimental and imaging

studies, which indicate that tasks tapping each system can be performed

simultaneously (Baddeley, 1986; Baddely, 1996; Cowan, 1997) and activate different regions of the frontal cortex (Jonides & Smith, 1997). In children, the existence of three separate components has also been supported using factor analytic studies (Gathercole, Pickering, Ambridge, & Wearing, 2004).

In general, it seems that this conceptualisation of the central executive is synonymous with executive function as a whole, with tasks such as the division and allocation of attention, inhibitory control and task-switching included in this

construct (Zoelch et al., 2005). Indeed, the idea that working memory, by definition of it’s limited capacity, necessarily inhibits information, has led some to propose that working memory is sufficient to explain executive function in general (Kimberg et al., 1997; Pennington, 1994). However, such explanations potentially replace one homuncular framework with another, and do not eliminate a need to determine the specific processes that working memory or executive function perform.

2.1.2 Attentional control

Another key aspect of executive function concerns the effective regulation of attention (Ruff & Rothbart, 1996). Studies of attentional networks in the brain indicate that there are 3 principal components of attention (Reuda, Posner, & Rothbart, 2004; Reuda, Posner, & Rothbart, 2005). These include arousal, the most basic form of attention, which is mediated by areas of the hindbrain (Posner, 2000). The second form of attention identified by neurological studies is orienting or selective attention, which involves selecting specific stimuli on which to focus from the myriad sensory stimuli we encounter (Reuda et al., 2004; Reuda et al., 2005). This form of attention is mediated by areas in the midbrain, specifically the striatum, as well as parietal and temporal regions. Finally, researchers refer to executive

attention, a more sophisticated form of attention that involves keeping a stimulus in mind and resisting interference, inhibiting responses that are inappropriate and sustaining attention over time (Anderson, 2002). This form of attention appears to rely on communicating regions of the anterior cingulate and the prefrontal cortex (Casey, 2000; Fuster, 2003; Posner, 2000). A key aspect that distinguishes executive attention from other attentional processes is that it involves effortful, volitional control. Aspects of executive attention that were selected for investigation in this study included sustained attention and selective attention (Cohen, Sparling-Cohen, & O'Donnel, 1993; Halperin, 1996; Stuss et al., 1995), as well as inhibitory control, which is described below.

2.1.3 Inhibitory Control

One aspect of executive attention is inhibitory control, or the capacity to control cognition by blocking out irrelevant stimuli (Hughes et al., 2004). Generally, tasks used to assess inhibitory control test the ability to inhibit a prepotent response, interrupt an ongoing response or resist distraction from interference. However, in the context of executive function, inhibition or inhibitory control is often assessed as the conscious suppression of a response that is learned or dominant (Miyake et al., 2000). The importance of inhibitory control for the maintenance of goal-directed behaviour is again illustrated in several case studies of patients with damage to the frontal areas of the brain. In these patients, behaviour is often perseverative (e.g. they will repeat a motoric action, such as shaking hands repeatedly), with irrelevant information often intruding (e.g. while making up stories, the patient will incorporate details about specific objects they are looking at at the time; Duncan, 1986).

Inhibitory control may be particularly relevant in the study of disorders of attention and may be a vital first step in allowing us to pause and activate other executive processes, such as working memory, self-regulation and internal dialogue (Barkley,

1997, 1998).

2.1.4 Shifting/Cognitive Flexibility

Set-shifting or cognitive flexibility is also encompassed in the latent construct of executive function. This is the ability to flexibly disengage from a strategy or stimulus that is not appropriate and apply a new one (Barkley, 1997, 1998). Typically, measures of shifting will involve the participant having to replace a learned or typical response with another, often less salient response. In this case, executive control is needed to determine which response is adaptive in the given situation, inhibit the less adaptive response and control responses in accordance with this decision (Ruff & Rothbart, 1996; van der Sluis et al., 2007). Some tests of set- shifting also involve mental abstraction, in that participants must use reward feedback to deduce a new behavioural requirement.

2.1.5 Planning and Self-Monitoring

Planning and problem-solving involve the breakdown and mental dissection of goals into smaller steps and then the re-synthesis of these steps to achieve the goal (Hughes et al., 2004). These processes are required when we have to reach a goal by overcoming obstacles. In order to do this, we must invoke rules or strategies and involve cause and effect principals that are often built up by experience. We also need to consistently evaluate our behaviour based on the goal we are trying to achieve (Bjorklund, 2005). Self-monitoring can include awareness of feedback, checking for mistakes in one’s work and self-pacing (Miller, 2005).