The Sensitive Periods of the Monkey’s Visual Cortex

Abstract

The mechanisms of differentiation and interaction of developing tissues remain deeply mysterious. Presumably, in most cases, the crucial events underlying the timing of developmental changes are determined by the expression of genes. In advanced mammals, most of the major structural developments, in the brain no less than in the rest of the body, take place before birth. However, although little if any multiplication of neurons occurs after birth, nerve cells certainly can continue to grow, differentiate and form new dendrites, axons and synapses (see Purves and Lichtman, 1985). Not only do these positive changes contribute to post-natal development of the brain but so do a number of important negative events. There is growing evidence that, in many parts of the nervous system, there is an initial overproduction of neurons and/or axonal connections, and that much of the refinement of connectivity in the developing brain involves the death of unwanted cells and the elimination of “inappropriate” axons and synapses (see Cowan et al., 1984). There are now several examples of such regressive developmental phenomena occurring postnatally.