Recent studies of sensory deprivation have shown that reduced stimulus-variability may appreciably modify sensory and cognitive efficiency. The McGill experiments demonstrated that exposing human Ss for several days to an environment, which minimized stimulus-change and restricted visual functions to the perception of diffuse light, resulted in progressive intellectual deterioration, hallucinations, and a variety of visual disturbances.1 Similarly, studies of visual monitoring have shown that impairments in attention increase with the length of the period of vigilance.2 Hebb has suggested that these disturbances in sensory efficiency result from the lack of stimulation of the nonspecific arousal system of the brain stem.3 According to his formulation, reduced exteroceptive stimulation may lead to a lowering of the generalized drive which presumably energizes the normal, perceptual functions. On the other hand, contrary results were obtained by Vernon and Hoffman who reported that periods of relatively complete sensory deprivation (up to 48 hrs., in a lightproof, soundproof, floating room) resulted in improvements in learning ability which correlated directly with the length of deprivation.4 Butler obtained similar results. He found an increase in the responsiveness of monkeys to visual incentives after periods of visual deprivation varying from 2-8 hrs. He ascribed his findings to the activation of a visual exploratory drive that increased with the duration of deprivation.5 The divergent results obtained in this area may be due to variations in the conditions of deprivation, or to differences in the measures of behavioral efficiency employed, or to both. In the present study, a comparison is made of the effects of two different conditions of sensory deprivation upon perceptual functioning in vision. Vernon and Hoffman's findings suggest that increments in visual efficiency would result from relatively complete visual deprivation.6 The visual exploratory drive, invoked by Butler, could account for the increments in visual functioning.7