Thalamic and cortical contributions to neural plasticity after limb amputation

Little is known about the substrates for the large-scale shifts in the cort ical representation produced by limb amputation. Subcortical changes likely contribute to the cortical remodeling, yet there is little data regarding the extent and pattern of reorganization in thalamus after such a massive d eafferentation. Moreover, the relationship between changes in thalamus and in cortex after injuries of this nature is virtually unexplored. Multiunit microelectrode maps were made in the somatosensory thalamus and cortex of t wo monkeys that had long-standing, accidental forelimb amputations. In the deprived portion of the ventroposterior nucleus of the thalamus (VP), where stimulation to the hand would normally activate neurons, new receptive fie lds had emerged. At some recording sites within the deprived zone of VP, ne urons responded to stimulation of the remaining stump of the arm and at oth er sites neurons responded to stimulation of both the stump and the face. T his same overall pattern of reorganization was present in the deprived hand representation of cortical area 3b. Thus thalamic changes produced by limb amputation appear to be an important substrate of cortical reorganization. However, a decrease in the frequency of abnormal stump/face fields in area 3b compared with VP and a reduction in the size of the fields suggests tha t cortical mechanisms of plasticity may refine the information relayed from thalamus.