Dynamic regulation of receptive fields and maps in the adult sensory cortex.

A dominant belief in neuroscience is that sensory systems in the adult are stable, in contrast to the extensive and pervasive plasticity that characterizes development of the nervous system. Empirical bases for this dogma of sensory immutability include the usually precise and stable responses of sensory neurons in anesthetized animals and the reduction of sensory cortical plasticity beyond critical periods of development. The subjective experience of neuro-scientists also supports the dogma. Perception of the outside world appears to be clear, immediate and effortless. To most workers, this implies that the sensory systems, once having developed, must be stable in order to provide accurate information about the environment. However, a rapidly growing literature attests to a very large degree of short- and long-term modification of receptive fields (RF) and reorganization of representational maps under variety of circumstances: learning, sensory stimulation, and sensory deafferentation. This chapter reviews contemporary findings concerning the dynamic regulation of receptive fields and maps in the primary auditory, somatosensory, and visual cortices of the adult brain. In contrast to previous reviews that have been confined largely to the perspective of sensory physiology, this article also emphasizes behavioral considerations. This seems to be appropriate, if not mandatory, because behavior is normally dynamic and adaptive, and sensory cortex is notable for its evolutionary development and implication in higher functions. The present coverage is highly selective, necessitated by severe constraints of space. Detailed analyses of publications were not possible and coverage of the effects of sensory deafferentation had to be limited to a scant summary of major effects and their possible relevance to sensory stimulation and learning; fortunately the effects of deafferentation have been reviewed in detail (Kaas 1991). Within the literature on sensory stimulation and learning, studies limited to standard learning paradigms were not included, in favor of studies of receptive fields and representational maps. These limitations should not unduly compromise this review because its intention is mainly conceptual. Specifically, the goal is to provide a framework that will be useful for thinking about both current and future research on adult sensory cortical plasticity and reorganization. This framework is based on an empirical law that is not yet widely appreciated in neuroscience. It may be summarized as follows: Behaving (i.e. waking) animals can continually acquire and retain information about (a) individual sensory stimuli, (b) relationships between various sensory stimuli, and (c) relationships between their own behavior and its sensory consequences. An implication of this law is that the attainment of an adequate understanding of how sensory cortex in the adult subserves perception and behavior also requires achieving an adequate account of the role of learning in sensory cortex. In theory, this role could have been nil. In fact it is not, as attested by the results of explicit learning experiments and other studies that can reasonably be considered to involve learning. The role of learning in denervation-induced plasticity and reorganization is currently largely conjectural but cannot be discounted. The following topics are discussed in turn: the relationship between sensory physiology and learning, basic forms of learning that are particularly relevant to adult cortical plasticity, methodological considerations, major issues and emerging principles in learning and sensory cortex, examples of these principles from the literature on learning, brief comments on the effects of sensory deafferentation, and conclusions.