Sunday, May 06, 2012

Dangerous Child: Critical and Sensitive Periods of Plasticity

The term "neural plasticity" means the ability of the brain to reshape itself. Critical periods of brain plasticity are times when particular circuits and intercircuits of the brain are particularly prepared for experiences which will assist the genetically encoded development of those circuits.

The brain tends to develop from posterior to anterior. From the occipital lobe in infancy to the prefrontal lobes in late adolescence and early adulthood, brain circuits mature and myelinate according to a particular sequence which is genetically encoded -- but can be altered somewhat by experience.

If a newborn's eyelids are sewn shut so that he cannot see from the time of birth, his occipital lobes will eventually be used for other types of processing rather than seeing. If only one eye is unable to see, the other eye's visual input will move into the brain territory which would have been used for the "dark eye's" input.

More about what is known scientifically about critical periods, with an emphasis on the visual system:
From polyglots to virtuosi, human performance reflects the neural circuits that are laid down by early experience. Although learning is possible throughout life, there is no doubt that those who start younger fare better, and that plasticity is enhanced during specific windows of opportunity. An understanding of the neural basis of such CRITICAL or SENSITIVE PERIODS of brain development would inform not only classroom and educational policy, but also drug design, clinical therapy and strategies for improved learning into adulthood. Although which might be the critical periods for higher cognitive functions such as language, music or emotional control is the subject of popular debate, such sweeping questions fail to acknowledge the sequential nature of a multistage process that involves many brain regions. _Critical Periods in Local Cortical Circuits (PDF)

Critical Periods in Language Acquisition (PDF)

Much of the knowledge about critical and sensitive developmental periods of plasticity was learned from animal research. Here is an intriguing study demonstrating the restoration of critical period plasticity in the auditory cortex of rats (PDF).

The concept of "critical periods" is quite controversial. Perhaps one reason for the controversy is that many scientists do not want to consider that very young children may have special needs which are not easily met except by persons who are heavily invested in that child. Many child psychologists are women who in fact were unable to take time away from their careers to spend intense time with a child who may have been passing through several critical periods. Subconsciously, such a scientist might wish to minimise any blame to herself for pursuing her career -- even if the only person who might possibly point a finger is herself.

But careful research in animals has clearly demonstrated that animals raised in an environmentally complex -- stimulus rich -- environment, experience superior neural and brain support structure development than animals raised in a stimulus poor environment. It is not likely that the developing brains of human infants are an exception to this tendency to thrive on the richness of stimuli in the environment.

We are accustomed to hearing -- in regard to aging and memory -- "use it or lose it!" But that maxim is likely to apply in a much deeper manner to the developmental time windows in the young brain.

But there is a problem, in that very few neuroscientists, cognitive scientists, or child and adolescent developmental specialists actually understand how the mature brain works, much less how the working brain came to be the way it is through various developmental periods.

It is easily possible for an interested and intelligent parent to know far more about the natural development of the child than most "experts", through observation, careful reading, and trial and error. And if a parent wants his child to develop into a "dangerous child," the parent will need to work hard to understand the process -- preferably before the child reaches each critical period.

It may seem a bit unprofessional to think of a child's developing brain in these terms, but in many ways a child's developing brain is much like a fine gourmet dish, or a carefully prepared perfume. The sequence of assembly is crucial, as is the skillful touch applied to each step, each finely textured layer.

Of course, the developing brain is undergoing many active processes simultaneously, and is not a passive recipient of "the master's touch." Brains are capable of turning out rather well in spite of what seem like a large number of stupid mistakes on the part of caregivers, parents, teachers, and society. But that is no excuse for being sloppy or negligent.

We will look at critical periods more, and at the related concept of "rites of passage."

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