European zyxwvutsrqp Journal of Neuroscience, zyxwvutsrqpo Vol. 6, pp. zyxwvutsrq 967-972, 1994 zyxwvut 0 European Neuroscience Association zy Depth Perception in Monocularly Deprived Cats Following Part-time Reverse Occlusion zyx Donald E. Mitchell', Maurice Ptito2 and Franco Lepore2 'Psychology Department, Dalhousie University, Halifax, Nova Scotia B3H 4J1, Canada 'Groupe de recherche en Neuropsychologie Experimentale, Universite de Montreal, Montreal, Canada Key words: stereopsis, visual development, amblyopia, critical period Abstract The behavioural effects of an early period of monocular deprivation can be extremely profound. However, it is possible to achieve a high degree of recovery, even to normal levels of visual acuity, by prompt imposition of certain regimes of part-time reverse occlusion where the initially non-deprived eye is occluded for only part of each day in order to allow a daily period of binocular visual exposure. In this paper we report on the depth perception of five monocularly deprived cats that had recovered normal visual acuity in both eyes following imposition of certain of the above occlusion regimes. Although three of the animals exhibited five- to sevenfold superiority of binocular over monocular depth thresholds, subsequent tests made on two of the animals revealed that they were unable to make stereoscopic discriminations with random-dot stereograms. Despite the recovery of normal visual acuity in both eyes, we conclude that these animals recover at best only local stereopsis. Introduction zyxwvutsrqpon The discovery of various critical periods during postnatal development of the visual cortex has been one of the most important findings to emerge from the extensive work that has been conducted in the last 25 years on the development of the mammalian central visual pathways. During these critical periods, the anatomical and physiological development of the visual system can be altered in a dramatic and sometimes permanent fashion by unusual early visual input (Movshon and van Sluyters, 1981; Frkgnac and Imbert, 1984; Mitchell and Timney, 1984; Rauschecker, 1991). However, on a more positive note, it has also been demonstrated that these critical periods are times where it may be possible, by prompt restoration of normal visual input, to reverse, either completely or partially, the deleterious effects of a previous period of early visual deprivation (e.g. Movshon, 1976; Blakemore et af., 1981). These discoveries have provided important insights into the rules that govern the development of the central visual pathways, and in so doing have cast new light on both the origins of the various forms of amblyopia in humans, as well as providing the impetus for initiation of treatment of the condition at a much earlier age than was the practice in the past. Much of what has been learned about the timing of sensitive periods in visual development has been derived from studies of the anatomical, physiological and behavioural effects of early periods of monocular deprivation imposed on kittens and infant monkeys (Movshon and Van Sluyters, 1981; Mitchell and Timney, 1984). The fact that the behavioural consequences of monocular deprivation mimic very closely many of the perceptual deficits experienced by amblyopic humans, led to the widespread use of this and closely allied forms of deprivation zyxwvut as a means for producing an animal model of deprivation amblyopia. Moreover, the use of this animal model was strengthened by the demonstration that many of the consequences of an early period of monocular deprivation could be to some extent reversed by a subsequent period of occlusion of the initially non-deprived eye (reverse occlusion), a procedure that bears a close resemblance to occlusion therapy for amblyopia. A particularly insightful example of the success that has resulted from prompt intervention in amblyopia is provided by the many reports of good eventual visual outcome associated with early treatment of congenital cataract (e.g. Lewis et al., 1986; Birch and Stager, 1988; Robb et af., 1988). Whereas it was once believed that deep amblyopia was an inevitable consequence of monocular cataract, it has been shown in recent years that early removal of the crystalline lens accompanied by meticulous optical correction of the resulting aphakia and careful adherence to certain regimes of patching of the normal eye can result in recovery of good vision in the formerly cataractous eye. Nevertheless, only a small proportion of patients that acquire good vision in both eyes as a result of such early treatment have been reported to subsequently develop even crude stereopsis (Tytla et al., 1993). A recent report (Gregg and Parks, 1992) of a single patient on whom cataract extraction was performed on the first day of life and who acquired a stereoacuity of 50 s at 8 years of age suggests that very prompt intervention may permit the acquisition of good stereopsis. In an effort to establish conditions that could lead to the development of better occlusion paradigms for treatment of amblyopia than those that exist at present, a systematic study was initiated over a decade ago of the consequences for the vision of the two eyes of different regimes of both part-time and full-time occlusion of the formerly non-deprived eye of monocularly deprived kittens. (Mitchell et af., 1984; Mitchell, 1991). Whereas good visual acuity was achieved in both eyes as an eventual Correspondence to: Dr D. E. Mitchell, as above Received 12 July 1993, revised 12 October 1993, accepted 21 January 1994