Functional reorganization in somatosensory cortical areas 3b and 1 of adult monkeys after median nerve repair: possible relationships to sensory recovery in humans

Previous studies have shown that the primary somatosensory cortex of adult mammals undergoes somatotopic reorganization in response to peripheral nerve transection. The present study assesses how cortical organization is affected when a transected nerve subsequently regenerates. The median nerve to one hand of adult owl monkeys was transected and repaired. Following nerve regeneration, the representations of the hand in cortical areas 3b and 1 were studied with neurophysiological mapping methods. The major results were as follows: Peripherally, median nerve transection, repair, and regeneration resulted in reinnervation of the median nerve skin territory. Centrally, both the initial loss and subsequent regeneration of median nerve inputs caused reorganizational changes in cortex. Reorganizational changes were specifically restricted to regions of the hand cortex where inputs from the median nerve were normally represented. The functional features of cortical regions that recovered tactile responsiveness from reinnervated skin regions were abnormal in several respects. Most notably, these regions contained recording sites with abnormally located or multiple cutaneous receptive fields, and contained major topographical changes, such as reestablishment of palmar pad or digit representations in small, discontinuous patches of cortex. Normal organizational features were reestablished to a more limited extent. These features included recovery of delimited, discrete receptive fields and reestablishment of topographic representations for localized skin areas. Different transformations in topographical organization were seen in areas 3b and 1 of the same monkey. These results suggest that nerve regeneration reestablishes the cortical capacity to process tactile information from reinnervated skin via a prolonged reorganizational process that appears dependent on peripheral and central factors. Cortical recovery mechanisms clearly appear to have limitations, since preinjury patterns of cortical organization are not widely recovered even almost 1 year after repair. We suggest possible relationships between cortical reorganizational changes in these primates, and postrepair sensory changes in humans.