Inhibitory control and brain activation
Casey et al. (1997) used an fMRI scanner to examine patterns of brain activation during a Go/NoGo task. Nine children and nine adults took part. The children were between the ages of 7 and 12 years, and the adults were between the ages of 21 and 24 years.

The participants undertook a Go/NoGo task that involved responding to any letter that was presented to them on a screen inside the scanner, except for the letter X. Functional MRI scanners are like a large, narrow tube into which the participant is slid on their back, so a hand-held device was specially constructed to record the participants’ responses.

The researchers found that during the task adults and children showed the same location of brain activation within the prefrontal cortex. However, they also observed that the amount of activation was significantly greater for the children. The interpretation of this finding is complex, but it is likely to relate to the fact that the task places more demands on executive functions for children than for adults.

For adults the executive demands of the task are probably lower. This interpretation is supported by the fact that the adults performed better than the children on the task. Soas suggested in earlier sections of this chapter, and in Chapter 3, Section 5, increasing skill in a task is associated with decreasing involvement of the prefrontal cortex and executive function. The association between the prefrontal cortex and executive function is shown in the correlation reported by the researchers between levels of prefrontal activity and success on the task. Specifically it was found that increasing levels of prefrontal activity were associated with greater accuracy of performance (fewer errors of commission).

Inhibitory control in child development
The picture of development that has been built up in this section is one that shows the child gradually mastering the inhibitory control component of executive function. Development in this respect is already detectable in infancy and continues well into the school years.

Although it is not sensible to ask at what age this development is ‘complete’ (development does not just stop when a child becomes an adult), we can ask at what age it becomes impossible to distinguish between child and adult performance on standardized measures. In this respect Chelune and Baer (1986)have reported a steady improvement in performance on the Wisconsin Card Sorting Test from 6 years of age, with participants achieving adult levels of performance by around 10 years of age. This finding has been replicated in subsequent studies (for example, Levin et al., 1991; Welsh et al.,1991).

Executive function plays a crucial role in the early stages of mastering new skills. When a child is learning to read, the executive demands of the activity are high. A great deal of conscious effort is required on the part of the novice reader simply to decode the written symbols into words. So much conscious effort is required that, as was noted earlier, there may be insufficient cognitive resources to interpret the meaning of the text that is being read. Gradually, as reading skill increases, the executive demands of decoding diminish. The child becomes able to read the words ‘automatically’ and can allocate more resources to the business of constructing meanings. If you consider the development of any skill you will see that the involvement of executive function is greatest when the actions involved are still novel. As mastery of the skill develops,so the role of executive function diminishes, and automated action takes over.There is something of a paradox here: executive function is least well developedin the very people who need it most in order to develop new skills – young children.

As executive function develops, so children’s abilities to learn new skills improve, and they are increasingly able to behave in a planned, strategic and organized manner. They are able to stay ‘on-task’ longer. They are able, when necessary, to override habitual responses to prepotent stimuli. They become more skilled and flexible in ‘orchestrating’ elements of their thinking and behaviour, and they are able to engage in increasingly sophisticated planning and decision-making. Inhibitory control is only one component of this developmental trajectory, but it is of fundamental importance. One way to assess this importance is to look at the implications for child development of a failure to develop typical levels of inhibitory control.