Development of Executive Function

There is evidence that these different aspects of executive function may develop at varying rates over childhood (Romine & Reynolds, 2005; Welsh &

Pennington, 1988). The emergence of rudimentary executive control can be seen in infancy. By the early age of 9 months, children have developed simple forms of mental representation, as shown by the fact that they can search for a hidden reward in a goal-directed way (Berk, 2006). However, this process can be easily disrupted by placing more obstacles in the child’s path. The A-not-B paradigm, originally employed by Piaget (Piaget, 1954), involves repeatedly hiding a reward in an “A’ location and then switching the reward, while the child is watching, to a “B” location. After a delay interval of approximately 10 seconds, children below 1 year of age will generally respond by searching at the incorrect A location (Diamond, 2002; Espy, 1999). It has been suggested that children below the age of 1 year generally have difficulty in maintaining a memory trace for extended periods of time (Welsh, Friedman, & Spieker, 2006). However, manipulations of the paradigm have shown that children make the AB error even when looking at the correct “B” location (Diamond, 1985). Thus, it seems that both working memory and inhibitory control may be integral for successful performance. When the A-not-B paradigm is altered by adding more search locations, increasing delay time and adding more barriers to obtaining the reward, children of 2 to 3 years find the switch trial difficult to

accomplish correctly. This suggests a progressive improvement in the cognitive mechanisms necessary for successful accomplishment of the task through infancy and early childhood (Espy, 1999; Stahl & Pry, 2005; Zelazo, Reznick, & Spinazzola, 1998).

A further important scaffold for later developments in executive function involves the emergence of symbolic understanding at around 18 months of age (Ruff & Rothbart, 1996). While children of younger ages are able to remember the

location of a toy or object, children of around 18 months are able to represent objects as they could be. At around 18 months of age, children also become more

self-aware. Thus, they can imagine the consequences of their own actions on their environment. The developments in language and symbolism seen during this period are integral for planning, self-monitoring and higher-level problem solving (Dennis, 1991; Ruff & Rothbart, 1996).

With these developments in mental representation and symbolic thought, children progressively become able to utilize more complex rules to guide goal- directed behaviour. These developments are illustrated in children’s ability to solve card-sorting paradigms. For example, Zelazo, Reznick, and Pignon (1995)

administered a card-sorting task to children aged 31-33 months. Rules for sorting were based on ‘if’ statements, such as, “If it is something you can ride, put it here” and, “If it is something that makes music, put it here.” However, the researchers also introduced a series of control paradigms to assist children’s performance. These included external motivators, memory cues and having children state, rather than enact, sorting rules. The researchers found that children’s performance across conditions was similar, apart from the condition where children had only to verbalise the rules, rather than utilising them to sort the cards themselves. In this group,

performance was significantly better. Children in other experimental conditions were likely to make perseverative errors by placing cards in a box they had previously used. This suggests that young children are unable to utilise rules or reconcile

competing response tendencies effectively to guide behaviour, even though they may be well aware of task requirements.

Important changes occur as children grow. In the Dimensional Change Card Sort (Zelazo et al., 1995), children are required to sort cards according to categories. Cards can be sorted in different ways according to different rules, e.g. shape, colour or size. Using this task, Zelazo and colleagues (Zelazo, 1996; Zelazo et al., 1995) have demonstrated that children are able to progressively sort shapes according to

more complex rules. Two year olds, for example, are likely to begin sorting cards according to the correct dimension, but soon after will relapse and sort cards

randomly. At approximately 3 years, preschoolers are able to consistently sort cards according to one dimension. When they are asked to sort the cards by a different dimension, however, they are generally unable to do so. As in the former study, children this age were able to state the new rule, but continued to sort by the old one. This is similar to the behaviour of individuals with frontal lobe injuries; they are able to state a rule, but appear unable to apply it (Fernandez-Duque, Baird, & Posner, 2000). At approximately 4 years of age, most children are able to flexibly adopt either rule and apply it when required.

Based on these findings, Zelazo (1996) has suggested that children are

progressively able to formulate new rules by combining subrules such as, “If sorting by the shape rule, put the flowers here and the rabbits here,” and “if sorting by the colour rule, put the red ones here and the blue ones here,” into a more complex, embedded rule; “If sorting by shape, put the rabbits here and the flowers here, but if sorting by colour, put the red ones here and the blue ones here.” The gradual

integration of these if-then scenarios has implications for higher level planning and problem solving.

Studies using simple paradigms to investigate children’s ability to overcome conflicting impulses have shown that this skill is fairly well developed by 7 years of age, especially for tasks that require the inhibition of motor responses (Reuda et al., 2005; Welsh, 2002). For example, children’s number of correct responses on the tapping task, a task that requires children to override a prepotent motoric response by performing the action that is opposite from the examiner, increases significantly between the ages of 3 and 4. Response latency also becomes much shorter on this task between 5 and 6 years of age (Diamond & Taylor, 1996). Similarly, children’s

ability to inhibit their actions on the ‘Simple Simon’ task improves between the ages of 3 and 5 years (Backen Jones, Rothbart, & Posner, 2003; Diamond, 2002). Further developments in more complex forms of inhibition of irrelevant cognitive

information continue into late childhood (Zoelch et al., 2005).

With basic inhibitory control, mental representation and rule-based direction of behaviour apparent by the time children enter the classroom, the period of 5 to 7 years represents a rapid growth period for more sophisticated executive function skills. Children of 6 years of age show simple forms of organized searching and planning, with these abilities improving between the ages of 6 and 8 years. For instance, Passler, Isaac, and Hynd (1985) found that the greatest period of developmental change in tasks assessing cognitive inhibition, perseveration and overcoming conflict occurred between 6 and 8 years of age. The group of 6 year olds (age range 5 years, 6 months to 6 years, five months) showed lower performance than the groups of 8, 10 and 12 years olds. Similarly, Welsh et al. (1991) documented significant improvements on a simple variation of the Tower of Hanoi (Simon, 1975) task and in visual search efficiency in a cross-sectional study, such that children’s performance reached the same level as the adult subjects between 5 and 6 years. The researchers surmised that this age corresponds with increased ability to resist

distractions and inhibit maladaptive responses.

More recently, Brocki and Bohlin (1999) examined increases in executive function between the ages of 6 and 13. As opposed to reviewing performances in these age groups on a number of different tests, these authors first completed a factor analysis of their results and evaluated age-related changes for 1) a disinhibition factor, comprising components of two go-no-go type tasks, 2) a speed/arousal factor, comprising reaction time and failure to respond on these tasks, and 3) a working memory/fluency factor, consisting of spatial and verbal working memory measures

and a variation of the Stroop Task (Golden, 1978). Analyses showed that the greatest improvement in children’s performance on the speed and working memory factors occurred between the ages of 6 and 7 years.

A recent meta-analysis (Romine & Reynolds, 2005) summarised 7 studies examining age related changes in executive abilities in children of 5 to 17 years. This study showed that the greatest increases in ability occurred between the ages of 5 to 8 years, with an average effect size (Cohen’s d) of 1.17 for age-related changes in performance across measures of planning, fluency, inhibition and set-shifting in this age bracket. Another review (Anderson, 2002) highlighted early to middle childhood as representing a period of rapid improvement in the ability to control attention, process information fluently and flexibly shift between dimensions.

Therefore, several studies converge to suggest that, building on the development of mental representation, simple inhibitory control, symbolic

understanding and the appreciation of complex if-then scenarios, the 5-7 year shift represents a major period of development in executive function. Such findings suggest that this will also be a time when developmental assessment is likely to reveal increasing individual differences in the development of these key executive skills and their manifestation as children enter the school environment.