Michael M. Merzenich
Coleman Lab. and Keck Ctr. for Integrative Neurosci., Univ. of California at San Francisco, P.O. Box 0732, San Francisco, CA 94143-0732
An important property of the cerebral cortex is its ability to reorganize itself in response to changing sensory environment throughout life. Representational ``maps'' of skin surfaces in the primary somatosensory cortex (SI) are remodeled after many forms of input alteration. Two questions were addressed in our recent plasticity experiment: Under what specific conditions are spatially distributed and time-varying sensory inputs (a) integrated, or (b) segregated in their representations by the dynamic mechanisms underlying cortical function? The results showed that the topographic map of skin surfaces in SI was dramatically remodeled in adult owl monkeys trained to discriminate two tactile stimuli applied to distal and proximal segments of their fingers in alternation. Subsequent mapping of area 3b revealed neural responses with either distal or proximal multiple-digit receptive fields, reflecting cortical integration of inputs from temporally coincidentally excited skin surfaces. By contrast, the cortical representations of temporally noncoincidentally stimulated skin surfaces were segregated from each other. These findings directly demonstrate that for learning induced plastic changes in cortical topographic maps, afferent input integration and segregation are dependent on stimulus coincidence, and that regularly practiced hand use results in a representation of its sensory surfaces in the brain that reflects that specific use.