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The most reliable feature of somatosensory plasticity is that reorganization nearly always involves shifts of representation among near neighbors in the somatotopic map. This is by no means a new idea,1-2 but as additional experimental data emerge, it is becoming apparent that neighbor relationships are not as simple as they seem. Specifically, near neighbors in the sensory representation do not all have equal access to the deprived territory. This is particularly apparent from deafferentation studies where patterns of sensory deprivation are graded from mild to severe. The data suggest that there is a hierarchy within the central maps that dictate which inputs are preferred. Moreover, there seems to be alternative possibilities depending on the pattern of denervation.

Another basic feature of plasticity in somatosensory systems is that the full potential for reorganization emerges over an extended time course. Rapidly induced changes are apparent immediately after sensory deprivation, but characteristically involve only limited extents of the sensory representation. Somewhat more extensive modifications appear gradually over the subsequent weeks after the sensory manipulation. Much later, further remodeling may occur, and these long-term changes can span large magnitudes of the sensory representation.

The mechanisms that subserve the remodeling constitute a third common characteristic of plasticity in the somatosensory system. Different mechanisms typically are associated with the various time courses of reorganization: rapid changes mediated by synaptic unmasking, more slowly developing changes attributed to NMDA receptor mechanisms, and late-emerging changes produced by new growth. Additionally, since the plasticity mechanisms act at multiple levels of the pathway, there is potential for modulation of somatosensory information at each of the stations.

In the present chapter, the evidence for these common features of plasticity in the somatosensory system will be reviewed. Literature on the effects of peripheral denervation will be emphasized, since in the majority of studies, plasticity in somatosensory system is induced by removal or silencing of sensory inputs. However, presumably the same principles apply to plasticity induced by behavioral training or after sensory enrichment. Also, since much of the plasticity research has been concentrated on cortex, the discussion will be devoted predominantly to effects of deafferentation at the cortical level. Finally, work on primates will be emphasized. It should be stressed that the patterns of reorganization that emerge in any given situation may reflect the history of the individual or features of emphasis unique to certain species. Thus, outcomes across individuals and species may differ, despite similarities in the basic processes that underlie the plasticity.

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