Visual perceptual learning: a sign of neural plasticity at early stages of visual processing.

Examples of perceptual learning in various visual tasks are briefly reviewed. In spite of the variety of the tasks and stimuli, in most of these examples the effects of learning are specific for stimulus parameters and retinal location and transfer interoccularly. Enduring practice effects can be acquired within a single experimental session and/or progressively from one session to the next one, often continuing to improve until thousands of trials have been performed. The consolidation of learning effects from one session to the next one may occur in the waking state as well as during a normal night sleep, but is strongly dependent on the type of sleep. Improvement in performance does not require that the subject is informed of the correctness of his/her responses, but needs attention to the task: learning does not take place for the stimulus attributes that are not attended to. All this suggests that visual perceptual learning involves plastic changes at early neural processing levels, which are dependent for their induction and consolidation on the general behavioural state of the subject, such as attentiveness and type of sleep.