When I started this blog, it was my plan to spend a lot of time talking about "child myths"-- common beliefs about child development that do not turn out to be supported by research evidence. This was what I had done in my book "Child Development: Myths and Misunderstandings" (Sage, 2009), where I commented on 51 beliefs that turn out to be largely mythical--- the "urban legends" of the child development field. Events in the real world and the blog world have distracted me from doing this, but today I'd like to examine a common belief about brain development that is only in small part based on empirical evidence.
The myth in question is the idea that all parts of human brain development show a high level of plasticity, or guidance by environmental events. Some proponents of this belief actually claim that events in the environment cause all development, rather than just guiding it. These people suggest that appropriate stimulation (like listening to certain sounds or using certain kinds of video games) can cause the brain to develop, no matter when in the lifespan the stimulation is used.
However, there are a number of facts about brain development that are ignored by people who assume that all brain functions and structures have high plasticity and can be shaped by environmental stimulation. One important issue is that different parts of the brain actually have different developmental mechanisms; for some, plasticity is high, for others, it is low. When plasticity is low, this means, by definition, that genetic factors order the development of that brain part, and the environment plays a relatively small role, except for providing general factors like good diet.
There is one area of the brain that has been shown to have high plasticity, that is, to have its development guided by experience. This area is the much-studied visual cortex. For animals like humans, some birds, and cats, there is a very important aspect of the visual cortex that must be "fine-tuned" by experience. This is the brain's response to images seen by the two eyes. This response to the partially-overlapping images is the basis of depth perception, without which driving a car safely would be difficult, and catching mice would be impossible.
Why can't this ability to interpret the images just be inborn? The reason is that the brain's response must take into account the distance between the eyes. This distance is smaller in the very young baby than it will be later on after the head grows more, and a brain response that worked at birth would be all wrong by the time the child was 15 months old. For a good outcome, the brain must be able to alter its connections over time, and it does this in response to many experiences with images the child sees.
The brain manages this by creating many extra connections between neurons (what neurologists call "exuberant" growth) and then "pruning" them by getting rid of the ones that are not being used. When there is normal visual experience, this culminates in having about half of the neurons from each eye go to each side of the brain, and overlapping areas represented thoroughly on both sides. Experimental work with cats has shown that if the cat is not allowed to use one eye, the other eye takes up 80% of the possible connections. Studies of human beings show the surprising fact that baby boys go through the pruning event later than baby girls do, apparently as a result of the high level of testosterone boys have at this point in development.
An important point about plasticity of the visual cortex is that there is a particular period in development when such guidance can happen, a "critical" or "sensitive" period. Allowing those cats to look with both eyes later on does not enable them to reverse the developmental changes that already occurred.
Some authors have jumped to, the conclusion that what is true about one part of the brain is true about all parts, and that what is true at one point in development is true at all points. But these are false analogies. The brain is a complex organ in which different parts have different functions and different developmental trajectories. They also have different factors guiding development and different schedules over which changing rules are applied. That is why the idea that all brain development is entirely dependent on environmental events is a myth. It is above all problematic to assume that sensory experiences can reverse development that has already occurred.
Of course, this statement does not suggest that children's development can proceed without experience. Both cognitive and emotional growth involve learning from experience and require appropriate experiences during particular time frames, for the best outcome. But, given our present ability to measure changes in the brain, we should not assume that these developmental events follow the same rules as development of depth perception.
