These complex structures and neurochemical processes that facilitate executive functions are not fully developed at birth. They are formed and gradually refined in a carefully timed sequence of events that begins early in fetal development, continues into infancy, and matures quite slowly throughout childhood and into adolescence and adulthood.
Although virtually all neurotransmitter systems are present in the cortex at birth, Francine Benes (2001) has shown that the systems for dopa-mine and norepinephrine are much slower to develop. These systems, crucial to executive functions, require much elaboration over the course of development, some of which continues at least into early adulthood and possibly beyond.
Another important but slowly evolving element is "myelination," the development of a protective surface coating that insulates the fibers that carry messages within the brain and elsewhere in the body. Myelin functions much as insulation around an electrical wire; it protects from "short circuits" and can enhance up to a hundredfold the speed with which messages can zip along the brain's networks, sometimes reaching transmission speeds of two hundred miles per hour. While most myelination of the human brain occurs before age two, the process continues well into the fourth decade of life. The more complex structures of the brain, those that exercise more central management functions, are not fully myelinated until considerably later than other brain structures that are less complex (Sampaio and Truwit 2001).
Although executive functions are slower to develop than many other aspects of brain function, foundations for their development are observable early in childhood. Adele Diamond and Colleen Taylor (1996) studied children three-and-a-half to seven years old who were asked to perform simple tasks that involved holding two pieces of information in mind at the same time while holding back or making a specific response. Results showed that between the ages of three-and-a-half and six years children improve significantly in their ability to hold two things in mind and in their ability to inhibit a strong response so that they can accomplish a task. Findings also showed that the older children were able to sustain their performance over more trials while many younger children who performed correctly at first were quick to give up following the directions. The foundation for executive functions is laid early, but full development of these linked abilities to regulate action, sustain attention, and modulate one's emotions takes many years.
Not all children of the same age are at the same point of development in their executive functions and not all reach the same point over the course of their overall development. The reasons for these differences, linked to aspects of the individual often referred to as "temperament," may rely on aspects of body chemistry that are variable across persons of the same age, though they tend to be relatively stable in any given individual across time.
In a similar manner, individuals differ in their inborn baseline levels of sensitivity to change and stress. Studies by Jerome Kagan and colleagues (1994, 2004) showed that in a sample of infants about 20 percent are born with a very low threshold for anxiety; they tend from the earliest months of life to respond with obvious distress and excessive behavioral inhibition when confronted with novel or stressful situations. These individuals are likely to be seen by others as exceptionally "sensitive" or "overly fearful" well into childhood and beyond. Work by Louis Schmidt and others (1999) indicates that these more socially reticent and anxious children tend to be less successful than age-mates at tasks requiring use of working memory and are more often off task. This may be due to these children being flooded with anxious emotions in ways that interfere with their attending to other stimuli and tasks.
In contrast, Nathan Fox and colleagues (2001) have reported on exuberant infants who are highly reactive and display positive emotions in response to novel situations and mild stress. These children are quicker to interact socially with others, are more active in exploring their environment, and show less fear in new situations. These temperamental characteristics, which are present in infancy and differ considerably from one individual to another, suggest what most parents of more than one child are quick to notice: infants seem to arrive in this world with neurochemical "wiring" that lays a foundation for how they respond to people and situations. Even within the same family, individual children can differ considerably along these dimensions. These individual differences in basic physiology, sensitivity to stress, intensity of engagement and of effort, and so on are likely to have considerable influence on the development of what will eventually become executive functions of the brain.
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