By Philip David Zelazo,
This is the first feature of a multi-part series on the topic of executive function. Dr. Zelazo is the Nancy M. and John E. Lindahl Professor at the Institute of Child Development, University of Minnesota.
As any parent knows, one of the most striking things about human babies is the fact that they are so completely dependent on their caregivers for support. But babies are also dependent in another, more subtle way: their behaviour is dependent on events, both internal and external, that are beyond their conscious control. Instead of acting, they react. Psychologists sometimes describe babies as “stimulus bound” and “impulsive.” Consider: when they are awake and alert, they may observe events in their environment, especially events involving people. But instead of choosing what to observe and when, it seems as if babies’ attention is captured by whatever happens to be the most salient thing at the time. They may cry, for example, in response to their growing hunger, but then stop to orient to a loud noise or even smile as soon as they see a friendly face, forgetting, for a while, their hunger.
Over the course of the first year, a remarkable transformation occurs. The dependent, reactive infant is somehow transformed into a willful toddler who may want something, such as a favoured toy, and persist in his efforts to obtain it. While still impulsive by his parents’ standards, a toddler is nonetheless able to keep a goal in mind and resist easy distraction.
Further changes occurring during the next few years are equally remarkable. The willful toddler becomes an articulate preschooler who can follow verbal rules (at least in principle) and think about other people’s points of view. Gradually, he becomes able to worry about the future and reminisce about the past.
How is it that human beings go from being dependent to being independent, from reacting to acting, from being at the mercy of bodily needs and salient external events to being able to exert deliberate, conscious control over their own thoughts and actions and emotion? What is it that is changing?
Scientists who study conscious control often refer to it as “executive function.” If this phrase evokes the image of a little CEO in the head, it is supposed to. An executive is someone who decides upon a course of action, issues commands by virtue of rank in a hierarchy, and ensures that the commands are implemented. Executive function, therefore, refers to the business of making decisions and carrying them out, as when one is deliberately trying to solve a problem.
Thinking about executive function in the context of problem-solving is revealing because it shows that executive function can be broken down into subfunctions. That is, in order to deliberately solve a problem, one needs to do several smaller things, in a specific sequence:
Represent the problem: “What do I need to accomplish? What is preventing me from accomplishing it?”
Come up with a plan for a solving the problem.
Actually execute or carry out that plan.
Evaluate the adequacy of the attempted solution.
Executive function involves the orchestration of these subfunctions.
As adults, we sometimes—lamentably, not always—manage to exhibit impressive levels of executive function. We decide to do something, such as lose weight, come up with a plan, such as going on a diet, and then act accordingly. And if we fail to achieve our objective, we may try another solution, such as exercise. Infants and children are a different story. Children, it seems, only gradually acquire the ability to act in such a deliberate, planful fashion. To be sure, there are dramatic improvements in executive function in infancy and early childhood. But even adolescents may be woefully impulsive, failing to consider the consequences of their actions as they engage in risky behaviour. Why does executive function take so long to develop?
The brain bases of executive function
One answer is that the key brain areas involved in executive function, the frontal lobes and in particular the prefrontal cortex, are among the last brain regions to mature. They continue to develop beyond adolescence and into adulthood. Psychologists have learned a lot about executive function by considering it in relation to the brain. In fact, historically, the very notion of executive function is derived from observations of the consequences of damage to the prefrontal cortex. One well-known case involved a man named Phineas Gage, who in the mid-1800s, suffered a horrible work-related accident while laying down railroad tracks: a six-foot tamping iron was blown through the front part of his head. Remarkably, Gage survived. Even more remarkably, many of his basic cognitive skills were preserved. Despite this, however, he became impulsive and grossly irresponsible and died under tragic circumstances
More recent studies of the effects of specific lesions involving particular parts of the prefrontal cortex, for example as a result of gunshot wounds or a stroke, have revealed the cluster of behavioural impairments that we now refer to as impaired executive function. These impairments, which are very diverse but nonetheless bear a kind of “family resemblance” to one another, include difficulties with decision-making, planning, cognitive flexibility, inhibition of competing responses, and monitoring one’s own actions. One widely used measure of executive function, which causes such patients considerable difficulty, is the Stroop Colour Word Task. For this test, people are shown colour words, for example the word “red”, printed in non-matching coloured ink, for example blue, and are required to name the colour of the ink. Use the quiz on the right to try the Stroop Task yourself.
While many of their skills remain intact, adult patients with damage to the prefrontal cortex and problems with executive function occasionally behave like children. For example, they may make puerile jokes and fail to keep secrets. The French neurologist François Lhermitte also noted that patients with lesions to the prefrontal cortex may become stimulus-bound and respond automatically to objects in a stereotypical fashion, just like children.
Lhermitte described the case of one patient who had been a nurse prior to suffering damage to the prefrontal cortex. During an interview with this patient, Lhermitte casually arranged a number of medical props in his office, such as a blood-pressure gauge and a tongue depressor. The patient proceeded to use the props on Dr. Lhermitte.
Understanding executive function and how it develops
Research on executive function in healthy children has revealed the mental processes involved in deliberate problem-solving. It has also allowed psychologists to describe the typical development of executive function. The results of this research have been important not only because they shed light on an important aspect of human function—conscious control—but also because they also allow us to understand various ways in which the development of executive function can go awry.
For example, although executive function can be understood in fairly domain-general terms, a distinction can be made between the development of relatively “hot” emotional aspects of executive function associated with one part of the prefrontal cortex, and the development of more purely cognitive, “cool” aspects associated with another part. Whereas cool executive function is likely to be elicited by relatively abstract, context-free problems, such as certain logic problems, or laboratory tasks in which one is asked to sort the same pictures first by shape and then by color, hot executive function is required for problems that involve the regulation of affect and motivation. It is generally elicited by problems about which one really cares, that is, problems that have real consequences for one’s wellbeing.
Difficulties with executive function appear to be a common consequence of many different developmental disorders (such as autism and Attention Deficit/Hyperactivity Disorder), but different developmental disorders may involve impairments in different aspects of executive function.
In next month’s installment, Dr. Zelazo reviews how executive function develops in infancy and early childhood and discusses how the major milestones relate to social interaction and readiness for school.