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Brain development is a complex mix of influences

by Sylvain Comeau

Nature or nurture? According to Barrie J. Frost, the brains of animals and humans do not make that choice in their development to adulthood. Both genetics and physical and social stimuli work in concert, and you can't have one without the other.

The Queen's University professor of psychology, biology and physiology and Max Bell Fellow of the Canadian Institute for Advanced Research gave the annual Science College lecture on October 22 in the Alumni Auditorium of the Henry F. Hall Building.

"When one looks at the total amount of information contained in the genetic code of any species, then compares it to the total amount of information required to account for an adult, competent organism of that species, there is a big mismatch," Frost said.

"In other words, a lot more information is required to make a competent adult brain than is contained in the genome. I think that most theoretical geneticists would agree with that.

"The rest of the information comes from the environment. For example, we know that the visual system is formed by the nature of light and an organism's interaction with the visual world. I would make the case that the social environment acts in the same way."

The brain integrates social stimuli in a process of destruction as well as creation.

"In brain development, there is an enormous amount of sculpting away of material, as well as the production of material," Frost explained. "When the brain of an embryo is complete in terms of the number of cells, there are many more neurons than what we end up with as competent adults. In the first few years of life, human babies lose about one-third of the neurons in their cortex."

The process is a kind of Darwinian natural selection within the nervous system.

"There is also an enormous profusion of connections between nerve cells -- circuits that are made and then discarded. They are not discarded at random; it is as though there is a fierce competition, which is like evolution itself. The fittest survive, and fitness here is measured by the sort of information they process. If that information is considered important, the correspondent connection will survive."

If a part of the nervous system turns out to be little-used, it will be eliminated in favour of another. Frost gave the example of astigmatism, an eye defect in which people have trouble seeing certain kinds of lines.

"There is one area of the cortex devoted to visual stimuli, and one specific area that has to do with seeing lines of all kinds -- vertical, horizontal, left and right. But if someone is born with an astigmatism, and it is not caught until they are eight or nine, they will still not see certain lines clearly even after their vision has been corrected.

"Why? Because of this competition in the brain. During the period of development, they haven't had vertical lines focused very often. So those particular neurons don't survive, or they have been greatly weakened."

Frost feels that the precise combination of genetics and environment in individuals lends itself to nearly endless variations.

"I find it very unsatisfying to say that we are all a wonderful triumph of nature and nurture. The view that I'm presenting is that some kinds of information must come from the genome, while other kinds have a more reliable source out there in the environment. Natural selection will develop the most adaptive mix of the two."

Copyright 1998 Concordia's Thursday Report.