REPRESENTATION OF TACTILE ROUGHNESS IN THALAMUS AND SOMATOSENSORY CORTEX

Neuronal responses were recorded in the thalamic ventroposterior lateral nucleus and primary and secondary somatosensory cortical areas of two rhesus monkeys performing a tactile discrimination task. The subjects actively stroked their fingertips over gratings that varied in groove width. Many cells in each location displayed average firing rates that incrementally reflected groove width dimensions (0.5-2.9 mm). Approximately 10% of cortical cells were more active to surfaces with narrower grooves, i.e., had negative graded response functions. All thalamic cells and similar to 50% of cortical cells with positive graded functions to gratings also showed increased responsiveness to contact force. Some cells also varied their activity with stroke speed. Many thalamic, a few primary somatosensory cortical cells, and no secondary somatosensory cortical cells showed periodic firing patterns that reflected the spatial-temporal frequency of stimulation. Responses to gratings of nearly every cell with negative graded responses and the remaining cortical cells with positive functions were independent of contact force and stroke velocity. The results only partially confirm predictions based on different models of texture perception that propose spatial, intensive, or cross modal neural codes. Negative graded response functions may require a form of spatial convergence across a cell's receptive field that has not previously been discussed by these models.