Sensitive and vulnerable periods in the development of the visual system.

In advanced mammals the visual system consists of a number of parallel channels for the efficient processing of different aspects of the visual scene. Much of the basic anatomical structure of the visual pathway is constructed before birth. A wave of maturation sweeps through the system, from the eye to the visual cortex, the correct formation of connections depending on precisely timed interactions between axons and their targets. Competition between growing axons (apparently dependent on spontaneous impulse activity in those axons), cell death (partly influenced by competition between those cells' axons), axon withdrawal, trophic interactions--these and other mechanisms play a part in constructing the visual pathway and laying down basic 'maps' of the visual field before birth. Disturbances in such processes might underlie disorders of the genesis of the nervous system. At the level of the visual cortex, synaptic plasticity continues after birth and may permit cortical neurons to refine their processing capacities on the basis of information provided by the visual environment. This makes the young animal vulnerable to disturbances of visual experience early in life, which can cause virtually irreversible deficits in stereoscopic vision, visual resolution and sensitivity to contrast (amblyopia) in adult life.