Early perceptual learning.

Until recently, it was commonly believed that within the early stages of sensory processing, the functional properties of neurons and the circuitry of sensory cortex are subject to experience early in cortical development but are fixed in adulthood. It is obvious, however, that some form of neural plasticity must exist well into adulthood, because we continue to be capable of adapting to experience and of learning to recognize new objects. One usually associates learning with the acquisition and storage of complex percepts, such as faces, which is generally believed to be an attribute of advanced stages of cortical processing. There is an accumulating body of evidence indicating that, quite to the contrary, even at the earliest stages of sensory processing, neuronal functional specificity is mutable and subject to experience. In this issue of the Proceedings, Polat and Sagi (1) report an important functional consequence of perceptual learning: Lateral interactions in visual space, an essential component of the integration of local features into a unified percept, can be induced to increase in spatial extent by training (see below). The focus of this review is psychophysical evidence for learning at early stages in sensory pathways, particularly visual. Most of the studies of this form of learning do not require giving subjects error feedback. Rather, there is improvement in performance simply as a result of repeating a perceptual discrimination task many times, which involves exposure to a stimulus and evaluation of a particular visual attribute. Early perceptual learning has been seen in various experiments for over a century. But there has been a flurry of recent studies showing that this form of implicit learning operates on various time scales, ranging from seconds to weeks, and indicating that its mechanism may be found in primary sensory cortex. In parallel with the advances in the psychophysical characterization of perceptual learning, anatomical and physiological studies in primary sensory areas indicate that the properties of neurons and functional architecture of cortex are capable of undergoing modification by experience.