The one major developmental difference lies in developmental timing. The human brain experiences a greater portion of its neurological maturation in utero, while at the same time having a more protracted period of postnatal maturation. A number of investigators have suggested that the development of brain and behavior in humans and other animals reflects canalized epigenetic processes. To understand the brain's structural and chemical responses to alterations in the environment, it is essential first to examine the normal curve of development. The lateral geniculate nucleus is a thalamic station for visual impulses; in the course of the study it, too, was examined for possible plasticity of response to an enriched environment. The cortical changes with ions were more marked if the rats lived in the enriched environmental conditions rather than in the nonenriched. This experiment shows the unusual sensitivity of the cerebral cortex to external environmental conditions such as air ions.