1. The rate and extent of recovery of vision was studied in the deprived eye of kittens that had been monocularly deprived from near birth for periods that lasted from 6 weeks to 18 months. Recovery was measured in the two situations, where either both eyes were open following the initial deprivation (binocular recovery), or else the non-deprived eye was occluded so as to force the animal to employ its deprived eye (reverse occlusion). 2. Measurements were made of the visual acuity of the deprived eye for gratings at frequent intervals during recovery by means of a simple behavioural technique. 3. The acuity that the deprived eye eventually attained declined with increasing length of deprivation in a manner that could be approximated by a simple exponential decay. Only minimal visual recovery was observed in animals deprived beyond 1 year of age; only two of six animals recovered sufficient vision to enable measurement of visual acuity. In general, animals that were reverse occluded recovered better vision than did those that had both eyes open during recovery. 4. The recovery of vision in the deprived eye of monocularly deprived kittens was compared to that observed following equivalent periods of two forms of binocular deprivation, namely dark-rearing and binocular eyelid suture. 5. The recovery from the two forms of binocular deprivation was quite different. Whereas the extent of recovery from dark-rearing was considerably greater than that observed after equivalent periods of monocular deprivation, the recovery of a limited sample of cats that were binocularly deprived by eyelid suture was worse. 6. These findings suggest that some plasticity remains in the visual pathways for a longer time than indicated by experiments that examine the physiological effects of monocular deprivation on various visual cortical structures.
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