What is ‘executive function’?
There are two main types of human action. One includes habitual behaviours, like driving along a well-known route, that involve automatic responses and need little or no effortful, conscious processing. The second type includes flexible, adaptive responses to new or difficult situations, such as driving in an unfamiliar city. Executive function is an umbrella term used to describe the processes underlying this second type of action. More specifically, executive function is needed in situations that involve:
- the learning of new skills
- planning and decision making
- error correction or troubleshooting
- initiating novel sequences of actions
- danger or technical difficulty
- conscious moment-to-moment control of behaviour
- the need to overcome strong habitual responses
The development of executive function in children
For much of the twentieth century, research on executive function centred almost exclusively on adults. This was mainly because the prefrontal cortex was thought to become functionally mature only late in development, around adolescence (Luria, 1973).
However, it has become increasingly clear that the onset of the development of executive function occurs much earlier than was previously thought. This has become apparent following the appearance of more appropriate tools for studying it at earlier ages. In this section we will look at the course of one aspect of early executive function development; inhibitory control.
We have seen in other chapters in this book that one aspect of child development concerns the progressive organization of children’s behaviour and experience. One factor that underpins this is a developing ability to inhibit responses to stimuli. Why is this so important?
Activity: The richness of the sensory world
This activity encourages you to reflect on the complexity of your everyday environment.
Consider your sensory world. Look around the environment you are in. Try to observe everything you can see. Then sit back, shut your eyes and listen to everything that you can hear.After that, focus on what is touching you. Attend to each part of your body in turn. What can you feel? It takes a few minutes to get the most out of this activity, so do not hurry it.
The richness and complexity of the assaults on your senses are considerable. However, it ishard to experience this complexity to its full extent because the mature human mind is soexpert at filtering out irrelevance, and building stimuli into simpler, more meaningful (andorganized) patterns. If this ‘filtering’ did not take place, and you gave equal weight to allincoming sensory information, it would be impossible to behave in anything but a chaotic manner.
Your current plan of action probably revolves around reading these words. In order to do this successfully you have to be able to ignore most of the sensory world around you. You have to be able to prioritize the meaningful stimuli – the type that makes up the words – and inhibit responses to irrelevant stimuli, in order to enact your plan of reading the current paragraph,and to achieve the goal of finishing it.
What if you were unable to do this? What if you could not give the words on the page any greater priority in cognition than the grain of the wood on the table on which your keyboard is resting? What if you were unable to inhibit responses to the sound of the computer fan as it whirrs in the background, or the feel of your feet on the carpet?
If you were unable to inhibit responses to stimuli that do not relate to the task that you have planned to do, then it would probably be impossible to complete it and achieve your goal. You would be drawn from one stimulus to another, in a haphazard fashion, and it would be impossible to undertake any coherently organized action.
This is a rather extreme way of conveying the point, but young children and people with executive function related disorders of inhibitory control do have difficulty in prioritizing their response to task-related stimuli, and do have difficulty in inhibiting responses to what are referred to as ‘prepotent’ stimuli.
A prepotent stimulus is a stimulus that draws a person’s attention towards it, and which seems to cause the person to behave in a particular way (the prepotent response). Prepotency is a very important feature of effective everyday functioning. It is to be hoped, for example, that a red traffic light would draw adriver’s attention towards it, and cause the driver to behave in a certain way. Thesight, smell and feel of the mother’s breast are the most likely prepotent stimuli for the young breast-feeding infant.
In the course of typical development it is possible to observe infants and young children being distracted by inappropriate prepotent stimuli. By ‘inappropriate’ we mean stimuli that are nothing to do with the child’s current plan of action. For example, one might observe an 8-month-old infant catch sight of a toy on the other side of the room and begin crawling towards it. It is clear to an observer that they are enacting a plan to get the toy, but halfway across the room the infant notices a scrap of paper on the floor.
This seems to ‘capture’ their behaviour and their attention. They pick it up, sit down and inspect it. The original plan is now lost and they have been catapulted onto another stream of behaviour, which might involve another plan, which might itself get interrupted by another prepotent stimulus, and so on and so forth. This executive function analysis of a familiar scene offers one explanation of why infant behaviour sometimes appears somewhat haphazard and disorganized to an adult onlooker – according to this view it is because executive functions are as yet undeveloped.
One aspect of child development that psychologists have become interested in, then, is the way in which children develop an ability to inhibit responses to stimuli that are nothing to do with their current plan of action. Put another way, this amounts to an ability to prioritize responses to task-relevant (as opposed to task-irrelevant) stimuli. When children begin to be able to do this, their behaviour becomes less haphazard, and progressively more strategic and organized.
Measuring the development of inhibitory control
Psychologists use a number of methods to measure the development of inhibitory control. One widely used technique is known as the Stroop task.
Activity: The Stroop task
This activity allows you to do the Stroop task and gives you a way of experiencing prepotency directly.
Now, out loud, and working as accurately and as a quickly as you can, call out the colour in which each word is written. What happens?
You should find that after a few words you start to get confused, wanting to call out the word that you are reading, rather than the colour of the ink in which it is written. The meaning associated with the word is acting as a powerful prepotent stimulus. You have to inhibit everything you have learned about words and their meaning in order to call out the colour of the ink. You could take two measures of performance from this, each of which would give some information about inhibitory control: speed of completion, and number of errors made.
Why do you think that this task is not suitable for children of, say, 3 years of age?
The Stroop task tends to be used with older children and adults because of the demands it makes on literacy skills, which in themselves are not a component of executive function. Indeed young children’s limitations with respect to language processing pose a problem in finding age-appropriate executive function tasks. Children’s levels of verbal comprehension may influence their overallperformance on tasks that have complex instructions or written stimuli, and this decreases the validity of such tasks as measures of executive function.Consequently, tests for young children need to be kept as simple as possible. The Handgame and the Knock/Tap game are good examples of sometasks that have been designed to minimize the importance of written language skills. (You may also like to try a similar test, What's the animal?)
The Handgame is a task that has the same basic structure as the Stroop task from Activity 4. The child must inhibit a prepotent response in order to execute a rule guided action. Specifically, the child is first asked to imitate two hand actions (making afist and pointing a finger). Then, in the conflict condition, children must make the opposite responses (making a fist when the experimenter points their finger and vice-versa). This involves:
- inhibiting the prepotent response to imitate; and
- performing an action guided by the rule ‘do the opposite of what the experimenter is doing’.
The measure of executive function that this task provides is the number of errors in the conflict condition. The task is based on work by Luria and has been used with preschool children. Other variants of this task include Luria’s Knock/Tap game (in which the child must knock when the experimenter taps the table and vice-versa), the Opposite Worlds task (in which school-aged children are asked to say ‘one’ when they see a ‘2’ and to say ‘two’ when they see a ‘1’), and the Day/Night Stroop task (Gerstadtet al., 1994) in which children are instructed to say the word ‘day’ whenshown a line drawing of the moon and stars, and ‘night’ when
shown a line drawing of the sun.
You should be able to see how these tasks relate conceptually to the Stroop task, and how they are better suited to minds that are not yet at the stage of having overlearned (automated) the ability to read words.
Measuring the development of inhibitory control in infancy is particularly challenging. One method that is used is the structured observation of infant performance on certain tasks. Inferences are then made from these observations about inhibitory control. An example of this is the work of Diamond (2002). They observed infants of 8–11 months performing problem-solving tasks such as retrieving an object from an open box. They concluded that 9–10 month-old infants show evidence of inhibitory control, and that even in this narrow age band there was already evidence of older infants maintaining their attention on-task longer than the younger infants.
Most work on inhibitory control in childhood has focused on children who are 3 years of age and over. A range of tasks has been developed that is suitable for use with children of different ages. One of these is the Go/NoGo task (Drewe,1975). In one version of the task different letters are displayed, one after the other, on a computer screen. The child is instructed to press the space bar as quickly as they can whenever a letter is flashed onto the screen (Go), except when that letter is an ‘X’ (NoGo). Errors of commission, when a child presses the space bar mistakenly in response to the letter ‘X’, indicate a failure to inhibit. A variant of this task uses pictures of planes with a cartoon bomb as the ‘NoGo’ stimulus (Rubia et al., 2001).Note that in the ‘Simon says’ game, ‘do this’ (without the preceding ‘Simon says’) is equivalent to a NoGo stimulus.
Using tests such as these researchers have found that there are significant improvements in task performance between the ages of 3 and 6 years. In a study by Mahone et al. (2001), 87 typically developing children completed a computerized Go/NoGo task. Even though the 3-year-old children managed the task with few omission or commission errors, the researchers noted a developmental trend across their sample: increasing age was associated with steady and significant improvements in performance.More complex inhibitory control functions are tapped by non-verbal Stroop tasks (for example, Luria’s Day/Night, Handgame, Knock/Tap). These require children not only to inhibit a response (as in the Go/NoGo task), but also to execute a rule-guided action. The majority of 3 year olds fail the Day/Night task (Gerstadt et al., 1994), the Handgame (Hughes, 1996; Hughes, 1998a and b) and the Knock/Tap game (Perner and Lang, 2002). However, the majority of 4 year olds pass these tasks. Thus significant improvements in both simple and complex inhibitory control are evident in the pre-school years.
Developmental improvements in inhibitory control also continue throughout
childhood, as demonstrated by findings from studies with school-aged children using the Go/NoGo tasks (for example, Manly et al., 2001). Interestingly, findings from a brain imaging study by Casey et al. (1997) that used functional magnetic resonance imaging suggest that children and adultsshow similar patterns of brain activation during the Go/NoGo task, and that this activation is in the prefrontal cortex (see Research summary below).
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.
- The ability to inhibit responses to prepotent stimuli is a prerequisite for planned, intelligent action.
- Some components of executive function are already developing in preschool children, and play a much greater role in early child development than previously thought.
- Adult levels of performance on a standard measure of executive function are achieved by around 10 years of age.
- Executive function plays a crucial role in the initial mastery of new skills, before the skills become automated.
Casey, B. J., Trainor, R. J., Orendi, J. L. et al. (1997) ‘A developmental functional MRI study of prefrontal activation during performance of a Go-No-Go task’, Journal of Cognitive Neuroscience, vol. 9, pp. 835–47.
Chelune, G. J. and Baer, R. A. (1986) ‘Developmental norms for the Wisconsin Card Sorting Test’, Journal of Clinical and Experimental Neuropsychology, vol. 8,pp. 219–28.
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Manly, T., Anderson, V., Nimmo-Smith, I., Turner, A., Watson, P., and Robertson, I. H. (2001) ‘The differential assessment of children’s attention: the Test of Everyday Attention for Children (TEA-Ch), normative sample and ADHD performance’, Journal of Child Psychology and Psychiatry, vol. 42, pp. 1065–81.
Perner, J. and Lang, B. (2002) ‘What causes 3-year olds’ difficulty on the dimensional change card sorting task?’, Infant and Child Development, Special Issue on Executive Functions and Development, vol. 11, pp. 93–106.
Rubia, K., Taylor, E., Smith, A., Oksanen, H., Overmeyer, S. and Newman, S.(2001) ‘Neuropsychological analyses of impulsiveness in childhood hyperactivity’, British Journal of Psychiatry, vol. 179, pp. 138–43.
This extract from course ED209 is © Open University 2005