Often described as the most complex structure in the known universe, the brain confers on humanity its wonderful richness and diversity. Above all, it permits us to adapt to the many different environments and conditions in which human beings thrive, and to learn new information, behaviors, and skills throughout our lives. Insofar as we experience it or interact with it, the environment produces biochemical changes in the nerve cells within our brains. The physical changes within the brain that are caused by experience are the basis of all long-term memory. The brain area in which we know most about plasticity during development is the visual system.
Genes are largely responsible for the initial wiring up of the brain.
Nerve fibers from the retina, the membrane at the back of the eye which is the receiving surface for the visual world, make the long journey to a way station in the middle of the brain, and from there another set of fibers make the journey to the very back of the brain, the primary visual cortex. There, preliminary connections are set up, but the fine tuning is dependent not simply on genes, but on use. As light excites nerve cells in the retina and they signal to the brain, those connections that are used effectively are strengthened and maintained. Those that are not used are pruned away. That is, in the case of the developing visual system, there is a critical time period during which the appropriate parts of the brain remain plastic.