Investigation of the functional correlates of reorganization within the human somatosensory cortex

Much work in animals and humans has demonstrated the existence of changes in topographic organization within the somatosensory cortex (SSC) after amputation or nerve injury. Afferent inputs from one area of skin are able to activate novel areas of cortex after amputation of an adjacent body part. We have investigated the functional consequences of this reorganization in a group of patients with nerve injury. Using the microneurographic technique of intraneural microstimulation (INMS) we stimulated groups of nerve fibres, within individual fascicles proximal to the nerve transection, with small electrical pulses. This enabled us to activate the deafferented cortex that had presumably undergone remodelling and study the conscious percepts described by the subjects. In 39 fascicles from 10 subjects, we found that the sensations evoked on INMS were no different from those reported previously by subjects with intact nerves. This finding suggests that such reorganization within the SSC has little effect on the function of deafferented cortical neurones or subcortical relay stations. In a separate set of experiments, INMS was performed in 16 nerve fascicles from an adjacent non-injured nerve or uninjured fascicle within a partially injured nerve. The sensations evoked by INMS in these experiments were also comparable to those obtained in normal subjects. This indicates that the expanded cortical representation of adjacent non-anaesthetic skin does not influence the cortical processing of afferent information. Taken together, these findings lead us to question the notion that reorganization of connections within the somatosensory cortex equates to a change in function. Whilst it may be advantageous that the human brain is not `hard-wired', neurophysiological proof of functional plasticity in the adult somatosensory system as a result of deafferentation is elusive.