MAINTENANCE OF A SOMATOTOPIC CORTICAL MAP IN THE FACE OF DIMINISHING THALAMOCORTICAL INPUTS

This study addresses the extent of divergence in the ascending somatos ensory pathways of primates, Divergence of inputs from a particular bo dy part at each successive synaptic step in these pathways results in a potential magnification of the representation of that body part in t he somatosensory cortex, so that the representation can be expanded wh en peripheral input from other parts is lost, as in nerve lesions or a mputations, Lesions of increasing size were placed in the representati on of a finger in the ventral posterior thalamic nucleus (VPL) of maca que monkeys, After a survival period of 1-5 weeks, area 3b of the soma tosensory cortex ipsilateral to the lesion was mapped physiologically, and the extent of the representation of the affected and adjacent fin gers was determined, Lesions affecting less than 30% of the thalamic V PL nucleus were without effect upon the cortical representation of the finger whose thalamic representation was at the center of the lesion, Lesions affecting about 35% of the VPL nucleus resulted in a shrinkag e of the cortical representation of the finger whose thalamic represen tation was lesioned, with concomitant expansion of the representations of adjacent fingers, Beyond 35-40%, the whole cortical representation of the hand became silent, These results suggest that divergence of b rainstem and thalamocortical projections, although normally not expres sed, are sufficiently great to maintain a representation after a major loss of inputs from the periphery, This is likely to be one mechanism of representational plasticity in the cerebral cortex.