Neuronal response properties within subregions of raccoon somatosensory cortex 1 week after digit amputation.

Multiple penetrations in the somatosensory cortex of three anesthetized raccoons 1 week following amputation of the fourth digit provided detailed information about somatotopy and neuronal responsiveness in the deafferented cortex. Recordings in a total of 601 penetrations (292 in deafferented cortex and 309 in the surrounding cortex) were compared with those from intact control animals described previously (Rasmusson et al., 1991). The level of spontaneous activity increased within the deafferented cortex, with 42% of the sites having high or moderate levels of spontaneous activity, in comparison with 18% in control animals. There was also an increase in the incidence of inhibitory responses to stimulation of adjacent digits (26% of the penetrations vs. 10% in control animals), confirming previous findings. These two variables, increased spontaneous activity and the presence of strong lateral inhibition, were highly correlated in individual penetrations. An unexpected finding was that the cortex representing the intact parts of forepaw was also disrupted with respect to these two measures, suggesting that amputation had an effect outside the deafferented region. In contrast, response properties that are more clearly a reflection of information processing in the dorsal column-medial lemniscal pathway (adaptation and threshold) were altered only within the deafferented region. The deafferented region was not homogeneous immediately after amputation, but consisted of a radically affected core region and a slightly affected fringe adjacent to the intact representations. This inhomogeneity had also been apparent with partial digit deafferentation, reported previously. The fringe, approximately 1 mm in width, may reflect overlapping projections from adjacent digits at one or more levels of the somatosensory pathway. Since the size of the fringe is similar to the maximum extent of reorganization found in other models of reorganization, the mechanisms of plasticity within this region may involve an unmasking of pre-existing synapses with slight modification in synaptic strength. However, the plasticity within the core region of the raccoon seen in these experiments, which may be 5 mm from nondeafferented cortex, requires more extensive changes, perhaps via polysynaptic pathways.