Synaptic potentials evoked by convergent somatosensory and corticocortical inputs in raccoon somatosensory cortex: Substrates for plasticity

'Unmasking' of weak synaptic connections has been suggested as a mechanism for the early changes in cortical topographic maps that follow alterations of sensory activity. For such a mechanism to operate, convergent sensory inputs must already exist in the normal cortex. We tested for topographic and cross-modality convergence in primary somatosensory cortex of raccoon. The representation of glabrous skin of forepaw digits was chosen because, even though it is dominated by inputs from the glabrous skin of a single digit, it nevertheless comes to respond to stimulation of other digits when, e.g., a digit is removed. Intracellular recordings were made from 109 neurons in the representation of glabrous skin of digit 4. Neurons were tested for somatosensory inputs with electrical and natural stimulation of digits. Excitatory postsynaptic potentials (EPSPs) were evoked in 100% of the neurons (109/109) by electrical stimulation of glabrous skin of digit 4, and in 79% (31 of 39) by vibrotactile stimulation. Glabrous skin of digit 4 was not the sole source of somatosensory inputs. A minority of neurons generated EPSPs after electrical stimulation of hairy skin of digit 4 (10 of 98 neurons, 10%). Electrical stimulation of digits 3 or 5 evoked EPSPs in 22 of 103 neurons (21%). Natural stimulation (vibrotactile or hair bending) was also effective in most of these latter cases (digit 3, 6/7; digit 5, 9/10). Intracortical microstimulation of the 'heterogeneous zone' was used to test for corticocortical connections to neurons in the glabrous zone. We found a higher proportion of corticocortical EPSPs among neurons that had convergent somatosensory inputs when compared to neurons having inputs only from digit 4. This suggests that a corticocortical pathway could be relaying some of the convergent somatosensory inputs. We conclude that convergent somatosensory and corticocortical EPSPs, present in normal brain and available for unmasking, could be substrates for reorganizations of cortical topographic maps.