The improvement of perceptual capabilities with training can offer important insight into the physiological basis of learning in the cerebral cortex. The rapid time course and ease with which some perceptual capabilities can improve suggest that learning is an integral part of normal perception. Electrophysiological and neuroimaging studies of sensory systems in the cortex suggest that the changes underlying perceptual learning can occur in a variety of areas and are likely to involve multiple mechanisms. In particular, recent psychological and physiological studies suggest that perceptual learning might often involve the task-specific suppression of signals that interfere with performance.