Behavioral evidence for prolonged sensitivity to effects of monocular deprivation in dark-reared cats

Abstract

A comparison was made of the behavioral and physiological consequences of periods of unilateral eye closure imposed on animals that had been reared from birth in total darkness until at least 4 mo. of age, with those observed in normally reared animals that had been monocularly deprived for similar periods of time at the same age. Kittens [2] were reared in total darkness from the time of natural eye opening throughout the conventionally defined critical period until they were either 4 or 6 mo. old. Immediately on introduction to an illuminated envrionment, 1 eye was deprived of pattern vison by eyelid suture for a period of either 1.5 or 3 mo. By means of a jumping stand, behavioral measurements were made of the development of the vison of the nonoccluded eye in terms of the visual acuity for square-wave gratings. Following periods of apparent blindness that lasted 13 or 28 days, depending on the period of deprivation, there was a gradual improvement in the vision of the nonoccluded eye over the course of the next few months. The animal that had been dark reared for 4 mo. eventually achieved normal acuity, while the other animal attained an acuity (5.7 cycles/degree) that was just below that of normally reared animals. On first testing the vision of the formerly occluded eye, both animals appeared to be functionally blind. Physiological recordings from the striate cortex of one of these animals shortly thereafter revealed that many more cells could be excited through the nondeprived eye than through the formerly occluded eye. Over the next few days the animal recovered some vision through its formerly occluded eye, but thereafter the improvement in vision was very slight. Neither animal recovered acuity with this eye in excess of 2.5 cycles/degree. The partial recovery in vision was accompanied by a significant physiological recovery from the situation that obtained immediately following termination of the period of monocular occlusion. The behavioral and physiological effects of a period of monocular occlusion imposed on dark-reared animals were very much more pronounced than those observed after similar periods of occlusion imposed on animals reared in illuminated surroundings, who suffer only a slight and very temporary reduction in visual acuity. Dark-reared kittens [2] were subjected to 3 mo. of monocular eyelid suture after either 1 or 2 mo. exposure to light. Both animals exhibited severe deficits in the acuity of the formerly deprived eye following termination of the period of occlusion. The visual cortex was apparently susceptible to the effects of monocular deprivation at least 2 mo. after the animals were introduced to the light. The period of susceptibility of the visual cortex to the effects of monocular deprivation is evidently extended in animals deprived of visual experience. Thus the degree to which the visual cortex is susceptible to modification by monocular occlusion is dependent not only on the animal''s age, but also on the nature of its early visual history.