By Philip David Zelazo, Ph
D
This is the fifth 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 we saw in earlier installments of this series, executive function refers to the cognitive skills that are used to solve
a problem:
Executive function emerges early in infancy and continues to develop well into adolescence. This development parallels the
development of the prefrontal cortex, a part of the brain that contributes to executive function by allowing us to reflect
on a situation and consider a range of options, as opposed to responding impulsively or out of habit.
Difficulties with executive function are typical in childhood and adolescence, but they are especially pronounced in children
who are diagnosed with disorders such as autism, Attention-Deficit/Hyperactivity Disorder (ADHD), Conduct Disorder, and phenylketonuria
(PKU) — even when PKU is detected early and treated by dietary interventions. Children with these disorders are especially
likely to display the kinds of problems discussed in earlier installments of this series. For example, they may know rules
and be able to repeat them, but nonetheless have considerable difficulty putting them into practice. They may also exhibit
problem behaviours such as physical aggression.
In many cases, difficulties with executive function may be even greater than one would expect based on a child's overall developmental
level, as represented, for example, by performance on an IQ test. This can be quite frustrating for parents and caregivers,
who may expect more from a child, given his or her intelligence, than he or she is able to provide.
| Prefrontal Cortex |
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| The prefrontal cortex is the most anterior part of the frontal lobe. |
Because executive develops so slowly and is associated with a part of the brain (the prefrontal cortex) that continues to
develop into adulthood, it is not surprising that impairments in executive function are associated with so many different
disorders with childhood onset. Executive function appears to be a fragile, complex developmental achievement that is vulnerable
to disruption from a variety of sources, ranging from genetic abnormalities to environmental stressors. Although the developmental
window of vulnerability is long, the age at which any disturbance occurs is important.
The long-term implications of early difficulties with executive function are only starting to be understood. One clue to the
nature of these implications comes from a comparison of the consequences of head injuries occurring during adulthood and head
injuries occurring during childhood. We saw earlier what happened to Phineas Gage, who had a tamping rod blown through the
ventromedial part of the prefrontal cortex in a work-related accident. Patients like Gage are often grossly insensitive to
the consequences of their behaviour—both for themselves and for others. For example, they may make disastrous financial decisions,
and have severe difficulty maintaining personal relationships. However, they are rarely violent, and they do not appear to
show impairments in moral reasoning. Perhaps this is because they are able to rely on rules of conduct worked out prior to
their injuries.
In contrast, children with comparable injuries often display significant impairments in moral reasoning and simple perspective
taking, and they seem more likely to have histories of violence and criminal activity. For example, the neurologist Bruce
Price and his colleagues (1990) described a patient, known by his initials, G. K., who sustained extensive damage to the ventromedial
prefrontal cortex as an infant. In addition to chronic impulsive and reckless behaviour, G. K. displayed a stunning lack of
regard for other people's perspectives. For example, “When restricted for inappropriate behaviour by a ward attendant, he
escaped from the locked psychiatric unit, scratched the attendant's car with broken glass, signed his own name, and re-entered
the ward. When confronted, he denied his involvement.”
Cases like these raise the intriguing possibility that the typical development of executive function in childhood may be crucial
not only for cognitive function, but also for social, emotional, and moral development. Early impairments in executive function
and damage to the prefrontal cortex may have cascading consequences for many aspects of children's development.
Executive function and behaviour problems
Thinking about different aspects of executive function may help us better to understand relatively common problem behaviours,
such as physical aggression. University of Montreal psychologist Richard Tremblay and his colleagues have shown that physical
aggression (such as hitting, biting, and scratching) normally peaks in early childhood, around two years of age, and then
declines over the course of the preschool years. Perhaps the development of executive function allows children to refrain
from responding aggressively. Indeed, failures of executive function at any step in the sequence of solving problems could
result in aggressive behaviour.
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Represent: Hostile cognitive biases
Some children may routinely respond aggressively because they persist in representing interpersonal conflicts in a biased
and inflexible fashion. They may persist in assuming that other children are behaving in a hostile fashion even when, in fact,
the other children are being playful.
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Plan: Failure to anticipate consequences
Some children may routinely respond aggressively because they fail to plan and anticipate the negative consequences of their
own aggressive behaviour. For instance, they may fail to realize that if they hit another child, the other child will hit
them back, and this will provoke an all-out fight.
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Execute: Failure to inhibit, despite intentionsSome children may routinely respond aggressively, even though they understand the rules that govern a social situation (for
example, “I shouldn't hit others” or “I should listen to my teacher who warned me about hitting other children”), because
of problems with rule use analogous to those observed in tasks such as the Dimensional Change Card Sort (DCCS).
For more information on the DCCS, please see What is executive function?
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Evaluate: Failure to detect punishment information or learn from punishment
Some children may routinely respond aggressively because they have difficulty evaluating the effects of their behaviour and
learning from feedback (for example, “The last time I hit someone, he hit me back, and we both got in trouble”).
These possibilities are not mutually exclusive; some children may respond aggressively for any or all of these reasons. But
thinking about executive function is helping us to understand that in many cases, behaviour problems such as physical aggression
are not solely “bottom-up” problems as in the case of children who simply have strong aggressive tendencies. Rather, there
may be problems with the kinds of “top-down” control processes associated with executive function.
Of course, in most children these problematic behaviours decline before their consequences are likely to be severe — for example,
before children become strong enough to cause serious damage. However, for a small percentage of children (about 5% of the
population), levels of physical aggression remain high. These may be children for whom the development of executive function
is atypical.
Executive function and disorders with childhood onset
There are considerable differences among the many disorders associated with impairments in executive function. For example,
children with autism and children with ADHD show very different sets of symptoms:
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Autism is a pervasive developmental disorder that affects more than five children per 10,000, most of whom are boys (about
five boys for every girl). Children with autism are diagnosed on the basis of social abnormalities, language abnormalities,
and restricted and stereotyped patterns of behaviour. The severity of symptoms varies widely, and while some individuals with
autism show high levels of intellectual function, many are mentally retarded.
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In contrast, ADHD is an externalizing disorder characterized by hyperactivity,
impulsivity, and distractibility. Recent estimates
suggest that about 3% to 7% of children suffer from ADHD, most of whom are boys (anywhere from two to nine boys for every
girl). Many children with ADHD also display behaviour problems that may be diagnosed as
Oppositional Defiant Disorder or Conduct
Disorder.
Although it may be accurate to say that both autism and ADHD are associated with difficulties in the conscious self-regulation
of thought, action, and emotion, they are very different disorders. It seems that each disorder is associated with problems
with particular aspects of executive function.
Both children with autism and children with ADHD have difficulty planning future actions, as measured by the Tower of Hanoi.
(This test and other tasks used to measure executive function are described at the end of this article.) However:
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Only children with ADHD seem to have special difficulty with tasks that require them to suppress overlearned responses, such
as the Stroop Task and the Stop Signal Task, a measure that has been used extensively by Drs. Rosemary Tannock and Russell
Schachar at The Hospital for Sick Children.
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In contrast, children with autism, but not children with ADHD, seem to have special difficulty with cognitive flexibility,
measured by card sorting tasks such as the Dimensional Change Card Sort (DCCS) and the Wisconsin Card Sorting Test.
Training executive function
Given that these children have difficulties with executive function, is it possible to train executive function and thereby
reduce the likelihood of problem behaviours? A growing body of research is investigating the possibility that the healthy
development of executive function can in fact be fostered.
So far, the prospects of training executive function seem good. Just as the brain develops in part through use, executive
function appears to improve when it is exercised.
One study, conducted by researchers in Australia, involved healthy preschoolers who failed a Go-NoGo task. These children
were then given two simple measures of executive function on each of three occasions, along with feedback indicating how well
they performed. Then they were retested on the Go-NoGo task. Children who were trained did much better on the retest than
children who were not trained.
Similar results were obtained by a group in Austria. In this study, children were trained on the DCCS. After several training
sessions, children not only did better on the DCCS but also did better on tasks requiring them to think about other people’s
perspectives.
If simple practice with feedback can improve executive function in typically developing children, it may be beneficial for
children with various disorders.
In the next and final installment, we’ll explore the possibility of training executive function in more detail.
The Stroop Task is described in Part 1 of this series, What is executive function? The DCCS is described in Part 2, The development of executive function in infancy and early childhood.
Go-NoGo Task
In a commonly used version of the Go-NoGo task, children are required to respond to one cue, called the “Go stimulus,” while
refraining from responding to another stimulus, called the “NoGo stimulus.” This task provides a measure of attention, flexibility
of responding, and the ability to withhold a response.
Stop-Signal Task
Children watch a computer monitor and see a series of X's and O's. They are told to press the key on a keyboard (that is,
an X or an O) that matches the letter that appears on the screen—EXCEPT when they hear a tone (the “stop signal”), in which
case children should not press either key. This task measures children's ability to stop a response that is already underway.
Tower of Hanoi
This task, which is based on a popular nineteenth-century puzzle, consists of several disks of varying size that are fitted
onto three pegs. In one version, children must transfer the disks from one peg to another, moving only one disk at a time
and without placing a larger disk on top of a smaller disk. The challenge is to solve the problem in the minimum number of
moves. Doing so requires planning (thinking ahead).
Wisconsin Card Sorting Test (WCST)
In this task, children are presented with stimulus cards that differ on various dimensions, and then children are shown individual
cards that match different stimulus cards on different dimensions. Children must figure out the rule for sorting each card
(for example, “Match by colour”), and the experimenter informs the child after each card whether the sorting is right or wrong.
After a certain number of correct responses, the target dimension is shifted, and the child must discover this new sorting
rule. This task provides a measure of hypothesis testing and flexibility.
