Adult plasticity in the visual system.

When visual cortical neurons in adult mammals are deprived of their normal afferent input from retinae, they are capable of acquiring new receptive fields by modifying the effectiveness of existing intrinsic connections, a basis for topographic map reorganization. To gain insights into the underlying mechanisms and functional significance of this adult plasticity, we measured the spatial limits and time course of retinotopic map reorganization. We also determined whether reactivated neurons exhibit normal receptive field properties. We found that virtually all units in the denervated zone of cortex acquired new receptive fields (i.e., there were no silent areas in the cortex) and map reorganization can take place within hours of deafferentation provided that retinal lesions are relatively small (< 5 degrees). Furthermore, after long periods of recovery, reactivated units exhibited strikingly normal selectivity to stimulus orientation, direction of movement, and spatial frequency if relatively high contrast stimuli were used. However, responsiveness of these neurons, measured in terms of the maximum response amplitude and the contrast threshold, was clearly reduced. Thus, contrary to traditional belief, the adult visual cortex is capable of exhibiting considerable plasticity, and reactivated neurons are capable of contributing to an analysis of a visual scene.