By Philip David Zelazo, Ph
D
This is the second 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.
Although it continues to develop well into adolescence, the first signs of executive function—the conscious control of thought,
action, or emotion—emerge as early as the end of the first year of life.
Babies are willful, to be sure, but do they really exhibit “conscious control”? The answer seems to be “yes.” When babies
are about eight months old, for example, they can usually be encouraged to search for hidden objects after a brief delay—a
form of “hide and seek.” That is, if a baby is playing with a doll and her father covers the doll with a cloth, the baby may
remove the cloth and retrieve the doll. This behaviour by itself suggests some degree of executive function because the baby
keeps the doll in mind and performs one action (removing the cloth) in order to perform another action (retrieving the doll).
The first action is consciously controlled to the extent that it is treated deliberately as a means to an end. Babies at this
age are able to act in light of a goal.
But of course, infants’ emerging executive function is still very fragile and easily disrupted. The limitations on infants’
executive function were well described by the famous Swiss scientist Jean Piaget, a pioneer in the study of child development
who made intensive observations of his own and other children.
In one demonstration, Piaget (1954) played hide-and-seek with his nine-month-old daughter, Lucienne, using two cloths separated
by about 10 centimetres. He first placed the doll under one cloth (call it “Location A”). This was easy. Lucienne retrieved
the doll immediately. Piaget then placed the doll under the other cloth (Location B) as Lucienne watched carefully. Remarkably,
Lucienne persisted in searching for the doll under the first cloth (A), committing what is known as an “A-not-B error.” Even
more remarkably, Lucienne sometimes looked intently at the correct location even while she reached erroneously to the old
location, as if she knew, on some level, where the doll was hidden.
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A-Not-B Task |
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Sequence of events involved in the “A-not-B error.” This is a standard two-location search task, in which an object is hidden
first at one location (A) and then at another (B). A baby watches an object being hidden at Location A; the baby searches
at A and finds the object; the object is then hidden at Location B while the baby watches; the baby searches at A. “Where’s
the car?!”
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Errors like this are described as “perseverative”: someone perseveres in behaving in way that is no longer appropriate, often
despite knowing what they are supposed to do. Perseverative errors are classic examples of failures of executive function. By observing these errors and watching how children eventually overcome them, researchers can track
the development of executive function even in children who are too young to speak.
Lucienne was by no means unusual: Hundreds of scientific studies have now shown that most babies between the ages of about
eight and 12 months of age will commit the A-not-B error under some circumstances. After about 12 months, babies are much
less likely to make this error in situations like the one just described. Instead, they keep a goal in mind, and they aren’t
as easily distracted, for example, by the sight of the initial location and a tendency to search there again.
Do older children make A-not-B errors?
Interestingly, however, researchers have been able to elicit A-not-B errors in older children, such as two-year-olds, by making
the search task harder. For example, in one task, toddlers are shown a hiding box. This box has an opaque front door that
can be raised to reveal the contents of the box. The base of the box is a sliding tray. First, the box is opened and the experimenter
shows the child that he is placing a piece of candy in one of five plastic bags at the back of the box. Then the experimenter
closes the front door and places a foam barrier in front of the box. To retrieve the candy, the child has to remove the barrier,
pull the tray, select one of the symbols, and pull the string. After hiding candy several times at one location (A), the experimenter
hides a piece of candy at a different location (B).
Even though children are shown exactly where the candy is hidden on each trial, two-year-olds have a tendency to persist in
searching at the initial location. Sometimes they persist for dozens of trials, and naturally, they get frustrated: On each
trial, they watch the candy being hidden at Location B, but they keep going back to Location A, and keep failing to get the
candy! Observing children persist in vain like this, it is difficult to avoid feeling sorry for them; they clearly want the
candy, but they just can’t get it together to keep the correct location in mind and choose that location instead of the one where they had found the candy initially.
These same two-year-olds would almost certainly search correctly at Location B in the relatively simple two-location task
that posed problems for Lucienne at nine months of age. However, by adding more hiding locations for a total of five, increasing
the number of steps required to retrieve the hidden object, and embedding the crucial decision (whether to select the old
symbol or the new one) within a series of steps, experimenters were able to tax the skills underlying toddlers’ executive
function, and elicit perseverative errors.
Over the course of the second year of life, children become increasingly skilled at orchestrating the cognitive processes
required to solve problems such as these. This occurs, to some extent, because children are starting to acquire language and
use language to regulate their behaviour. Naturally, finding a hidden object, even at the new B location, is a lot easier
if one is able to say to oneself something like, “It’s over there, at Location B.” As children get older, language comes to
play an increasingly conspicuous role in executive function and its development.
The development of rule use
Parents of two-year-olds will notice that children at this age sometimes (alas, not always) comply with verbal rules: requests
and directives. And there is also scientific evidence that children at this age are starting to keep verbal rules in mind
and use them to guide their behaviour.
For example, in one paradigm, two-and-a-half- and three-year-old children are shown two “target” pictures, such as a chair
and a bus, each of which is affixed to a sorting tray, and they are asked to sort a series of test cards into the trays according
to a relatively arbitrary pair of rules: “If I show you something that goes outside the house, like this bus, put it here.
But if I show you something that goes inside the house, like this chair, put it there.” They are then shown 10 pictures, such
as a television, a snowman, a refrigerator, and so on.
The two-and-a-half-year-olds start out just fine, but then they show a tendency to perseverate on just one of the two rules.
For example, they might sort the television and the snowman correctly but then put the refrigerator and all the rest of the
test cards in the same tray as the snowman. It’s as if they can follow a single simple directive, but struggle to keep two
rules in mind and chose between them.
In contrast, by three years of age, children typically do very well on this task. Unlike children just six months younger,
most three-year-olds easily remember the two rules, chose deliberately between them, and then use the appropriate rule to
control their sorting. Incidentally, when three-year-olds are shown the very same sets of pictures and simply asked to put
the test cards with the target cards they go with, they perform poorly. This confirms that they are really using the rules
when the rules are provided.
The deliberate use of two explicit verbal rules to control one’s behaviour is a clear example of executive function. The child
is no longer simply reacting to stimuli, such as some salient aspects of the test cards; nor is the child responding in a
rigid, stereotyped fashion (for example, putting all of the cards in the same box). Rather, the child is acting deliberately
and flexibly, and in light of a conscious plan.
Improvements in rule use between three and four years of age
Three-year-olds’ rule use is impressive when compared to the relatively poor performance of a typical two-year-old, but it
is still quite limited. Three-year-olds, for example, respond perseveratively on a more complex card sorting task called the
Dimensional Change Card Sort.
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Dimensional Change Card Sort Task |
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Perspectives on rules, from different levels of consciousness. Two-year-olds represent one rule, such as “flowers go here.”
Three-year-olds reflect on rules and can represent the contrast between two rules, such as “flowers go here” versus “cars
go there.” Four-year-olds reflect on their representation of the contrast between two rules, and as a result, they can consider
the relation between two incompatible pairs of rules, such as “shapes” versus “colours.”
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In the Dimensional Change Card Sort, children are shown two target cards, such as a red car and a blue flower, and asked to
sort a series of mismatching test cards (red flower, blue car) first according to one dimension and then according to the
other. For example, they may first be told to sort by colour (“Put the blue ones here; put the red ones there”) and then told
to switch and sort by shape (“Okay, now put the flowers here; put the cars there”). Regardless of which dimension is presented
first, three-year-olds typically continue to sort the cards by that first dimension despite being told the new rules on every
trial—they perseverate on the first dimension. Moreover, three-year-olds who perseverate on the first dimension are nonetheless
able to answer questions about the new rules. For example, when children who are supposed to be sorting by shape are asked, “Where do the cars go in the shape game? And where do the flowers go?” they almost always
answer correctly. However, if they are then told to go ahead and sort the cards according to these rules (“Okay, good, now
play the shape game. Where does this flower go?”), nearly all of them perseverate, sorting the red flower by colour. The children
answer an explicit question about the new rules, showing that they know these rules, but then they immediately persist in
using the old rules!
By four years of age, most children switch easily from sorting by one dimension to sorting by the other. It’s as if they know
immediately when they see the test cards that they know two different ways of sorting them (shape rules and colour rules),
and so they are alert to information from the experimenter concerning which rules to use. Instead of simply being stuck within
the shape game or the colour game, it is as if they can step back from these games, consider them both, and then choose between
them: “If I’m playing the colour game, then if I see a red flower, it goes over here; but if I’m playing the shape game, then red flowers go there.”
Changes underlying changes in executive function
According to one theory, Zelazo's Levels of Consciousness model, the progression in children’s rule use from two to three
to four years of age well illustrates an important general feature of the development of executive function: with development,
children are increasingly able to reflect on their own representations (for example, from just intending to do something,
to knowing that they are intending to do something, to knowing that they know this, and so on). And increases in the extent
to which children can reflect on their representations, at increasingly higher levels of consciousness, allow them to formulate
and use more complex sets of rules for regulating their behaviour.
Whereas a two-year-old can get her mind around a simple directive (“Put the red ones here”), a three-year-old is able to reflect
on this directive and consider it in contrast to another directive: “If it’s red, put it here; but if it’s blue, put it there.”
That is, three-year-olds formulate a pair of rules. Four-year-olds can go one step further: they consider two incompatible pairs of rules and formulate a more general
rule for choosing between them: “If it’s the colour game, then if it’s red, put it here…”
Each of these changes in children’s fundamental ability to reflect on their representations and formulate more complex rules
has implications for children’s behaviour across a wide range of situations, from reasoning about right and wrong, to understanding
mechanical events, to considering other people’s perspectives and predicting their behaviour. And together, these changes
provide children with the basic abilities that underlie the diverse set of skills they need as they prepare for the transition
to school.
The four-year-old mind is remarkably complex, and a typical four-year-old can solve an impressive range of problems. Indeed,
this range is even wider than the range of problems that can be solved by the adults of any other species, including our nearest
relatives, the bonobo, an intelligent ape with whom human beings share over 98% of their genetic material. But of course,
by human standards, four-year-olds are just getting started.
In next month’s installment, Dr. Zelazo reviews how executive function develops in later childhood and adolescence.
