The development of vision in cats after extended periods of dark-rearing.

The time course of development of visual acuity for square wave gratings was measured behaviourally in a number of cats that were reared in total darkness until they were either 4 (5 cats) or 6 (1 cat) months of age. Less extensive measurements were also made on animals reared in a similar manner until they were either 1 1/2 or 10 months old. Initially, all the animals appeared to be blind, but signs of vision became evident after periods of time in an illuminated environment that ranged from a few days, in the case of animals dark-reared for only 1 1/2 to 4 months, to 1 to 2 months for those animals that were deprived for 6 months or more. Thereafter, visual acuity as measured on a jumping stand progressively improved, reaching, in the case of animals deprived for 4 months, values that were comparable to those of normal animals (6.9 cycles/deg) after 4 months of exposure to light. The animal deprived for 6 months remained apparently blind for a month and eventually attained an acuity (5.7 cycles/deg) that was somewhat less than that of normal animals. The fact that such high acuities can be achieved after periods of binocular deprivation that extend throughout the classically defined "critical period" suggest that the effect of dark-rearing is somehow to impede the natural decline with age in the degree to which cortical neurones are susceptible to environmental modification.