Introduction

Studies of the perceptual effects of early selected visual deprivation have had at least three motives. Arguably, the primary motive has been to identify the functional consequences of the cortical changes induced by early, selected, visual deprivation imposed within defined critical periods in an animal's early life, and en passant, to help shed light on the relationship between the physiological properties of neurons within area 17 and perception. A related motive has been to provide data pertinent to the debate over the complex and, indeed, sometimes paradoxical issue (e.g., Pettigrew, 1978) of how and why periods of abnormal visual experience exert such dramatic effects on certain aspects of cortical physiology for a long time after adult characteristics are present. Finally, the third motive behind a subset of studies has been to arrive at an understanding of the origins of certain common developmental disorders affecting human vision. Of particular interest have been the three major forms of amblyopia, one associated with the presence of strabismus (strabismic amblyopia), one with anisometropia (anisometropic amblyopia), and the last with a severe peripheral impediment to form vision in one eye (stimulus-deprivation amblyopia).

Over time, the inquiry into experiential influences on visual cortical neurons and perception has focused on a limited number of forms of visual deprivation. As a starting point, there were studies of the consequences of binocular visual deprivation. In these studies, animals were either deprived of all visual input as a result of being reared in total darkness, or else they were deprived of patterned visual input in the two eyes by means of bilateral eyelid suture. As a group, the remaining forms of deprivation involved selected visual input biased to one extreme of the range within which each of the major visual response characteristics of cortical neurons (ocular dominance, orientation selectivity, and directional selectivity) are distributed in the normal adult striate cortex. With respect to ocular dominance, two forms of selected visual deprivation have been used widely in an attempt to alter the distribution of cortical ocular dominance in area 17. These are monocular deprivation, whereby one eye is deprived of either patterned or all visual input, and manipulations, such as strabismus or alternating monocular deprivation, which involve an attempt to reduce or eliminate simultaneous or concordant visual input to the two eyes. Attempts to manipulate the distribution of preferences of orientation or directionally selective neurons in area 17 have employed various procedures designed to reduce early visual input to a narrow range of the spectrum of contour orientations or directions of motion that would be encountered in the environment and represented in the visual cortex of normally reared animals. Studies of experiential influences on the development of the physiological representation of directional selectivity have employed an additional form of selected visual exposure—namely, stroboscopic rearing—that virtually eliminates direction-selective neurons in area 17.

The effects of these various forms of early selected visual exposure on perception are described for various species that have been studied extensively, namely, monkeys, cats, and, to a limited extent, rodents. Where possible, results are included for humans in whom the early visual input was either known to be, or could be inferred to have been, biased in a manner similar to that imposed experimentally on animals. However, to begin, it is first necessary to discuss the behavioral methods that have been employed in the context of the existence of critical periods for the various physiological effects of early visual deprivation.