Acute effects of permanent and temporary denervation of the flying fox thumb were examined to test the hypothesis that a large area of skin around the cutaneous receptive field of multiunits (MRF) at a locus in primary somatosensory cortex (SI) supplies viable inputs which can be rapidly unmasked by interruption of the dominant input from the area of the MRF. 2. The immediate effect of amputation of the thumb at loci where the original receptive field was entirely removed was to produce large MRFs on adjacent body areas (wrist, forearm, prowing, and finger membranes). Greatly expanded MRFs were also produced when amputation removed only part of the original MRF at a cortical locus. 3. The probable source of input to account for the new receptive fields is the extensive arborization of ascending projections within the somatosensory pathway, which supply a cortical locus with a potential input from a far larger area than is represented in its normal receptive field. The rapidity with which new or expanded fields are seen following denervation indicates that the normally unexpressed inputs around a receptive field are not only potential inputs but are inherently viable. Hence the most likely explanation for the results of this study is that the effect of the denervation is to disrupt an inhibitory influence that normally has the role of shaping the receptive field. 4. Temporary anesthesia of all or part of a MRF produced similar initial effects to amputation. When responsiveness returned to the locally anesthetized area (after 10-30 min), an expanded MRF persisted for a short time after which the boundaries of the MRF shrank. This rapid reversal suggests that a mechanistic rather than a plastic change is the basis for the acute effect of a small denervation on SI.
