Responses of infragranular neurons in the rat primary somatosensory cortex to forepaw and hindpaw tactile stimuli

Infragranular layers constitute the main output of the primary somatosensory cortex and represent an important stage of cortico-cortical and cortico-subcortical integration. We have previously used chronic multiple single-unit recordings to study the spatiotemporal structure of tactile responses of infragranular neurons within the forepaw cortical representation in rats [Tutunculer B, Foffani G, Himes BT, Moxon KA (2006) Structure of the excitatory receptive fields of infragranular forelimb neurons in the rat primary somatosensory cortex responding to touch. Cereb Cortex 16:791–810]. Here we extend our understanding of this structure by studying the overlap between the forepaw and hindpaw cortical representations. We recorded 204 responsive neurons in chronic experiments from eight anesthetized rats. Overall, only 23% of neurons responded exclusively to one paw, 52% of neurons responded to two paws, 19% of neurons responded to three paws, and 5% of neurons responded to all four paws. Quantitative measures of response magnitudes and latencies revealed the following main results. (1) The responses of forepaw neurons overall displayed greater magnitudes and shorter latencies than the responses of hindpaw neurons. (2) The responses to ipsilateral stimuli displayed smaller magnitudes, and longer—and more variable—latencies than the responses to contralateral stimuli. (3) The responses of forepaw neurons to hindpaw stimuli displayed smaller magnitudes and longer latencies than the responses to forepaw stimuli, whereas the responses of hindpaw neurons to forepaw stimuli displayed smaller magnitudes but similar latencies compared with the responses to hindpaw stimuli. These results show that the spatiotemporal structure of tactile responses of infragranular neurons extends across all four paws, and provide the basic architecture for studying physiological integration and pathophysiological reorganization of tactile information in the infragranular layers of the rat primary somatosensory cortex.