When such biochemical changes are of large enough magnitude, they turn genes on and off inside those cells as part of normal processes that go by the name of brain plasticity. These physical changes caused by experience within the brain are the basis of all long-term memory. During development, as these little tweaks add up, our brains get wired up one way or another. This sculpting occurs, of course, by the regulation of genes by the environment, which not only builds new connections but also may eliminate connections that go unused. The area in which we know most about plasticity in 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 or synapses that are used effectively are strengthened and maintained. Those that are not used are pruned away. A condition that occasionally arises in children is a congenital cataract, an opacity that blocks light from entering the eye.