PROLONGED viewing of a grating pattern produces striking "after-effects", involving changes in the detectability, apparent size, orientation and contrast of subsequently viewed gratings1-3. Studies of perceptual after-effects have been used to infer properties of neurones in the human visual cortex2,4,5 similar to those pattern-selective neurones whose sensitivities have been directly measured in the visual cortex of cats and monkeys6,7. Such inferences are based on two assumptions: first, that perceptual changes result from changes in the distribution of activity within the responding population of neurones; second, that the effect of adaptation on each neurone of the population is to reduce its sensitivity uniformly to all stimuli. The experimental results reported here support the first but challenge the second assumption, as they show that after adaptation to a particular grating the sensitivity of a single neurone to that grating may be reduced more than its sensitivity to other gratings.
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