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The interhemispheric connections of somatosensory cortex in the gray-headed flying fox (Pteropus poliocephalus) were examined. Injections of anatomical tracers were placed into five electrophysiologically identified somatosensory areas: the primary somatosensory area (SI or area 3b), the anterior parietal areas 3a and 1/2, and the lateral somatosensory areas SII (the secondary somatosensory area) and PV (pairetal ventral area). In two animals, the hemisphere opposite to that containing the injection sites was explored electrophysiologically to allow the details of the topography of interconnections to be assessed. Examination of the areal distribution of labeled cell bodies and/or axon terminals in cortex sectioned tangential to the pial surface revealed several consistent findings. First, the density of connections varied as a function of the body part representation injected. For example, the area 3b representation of the trunk and structures of the face are more densely interconnected than the representation of distal body parts (e.g., digit 1, D1). Second, callosal connections appear to be both matched and mismatched to the body part representations injected in the opposite hemisphere. For example, an injection of retrograde tracer into the trunk representation of area 3b revealed connections from the trunk representation in the opposite hemisphere, as well as from shoulder and forelimb/wing representations. Third, the same body part is differentially connected in different fields via the corpus callosum. For example, the D1 representation in area 3b in one hemisphere had no connections with the area 3b D1 representation in the opposite hemisphere, whereas the D1 representation in area 1/2 had relatively dense reciprocal connections with area 1/2 in the opposite hemisphere. Finally, there are callosal projections to fields other than the homotopic, contralateral field. For example, the D1 representation in area 1/2 projects to contralateral area 1/2, and also to area 3b and SII.

1. 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.

The somatosensory cortex of adult mammals has been shown to have a capacity to reorganize when inputs are removed by cutting afferent nerves or amputating a part of the body. The area of cortex that would normally respond to stimulation of the missing input can become responsive to inputs from other parts of the body surface. Although a few animals have been studied with repeat recording, no attempt has been made to follow the time-course of changes at cortical loci and the immediate effects of a small amputation have not been reported. We have followed the changes in response in the primary somatosensory cortex in the flying-fox following amputation of the single exposed digit on the forelimb. Immediately after amputation, neurons in the area of cortex receiving inputs from the missing digit were not silent but responded to stimulation of adjoining regions of the digit, hand, arm and wing. In the week following amputation, the enlarged receptive fields shrank until they covered only the skin around the amputation wound. The immediate response is interpreted as a removal of inhibition and the subsequent shrinking of the field may be due to re-establishment of the inhibitory balance in the affected cortex and its inputs.