REPRESENTATION OF TACTILE ROUGHNESS IN THALAMUS AND SOMATOSENSORY CORTEX

Neuronal responses were recorded in the thalamic ventroposterior later al nucleus and primary and secondary somatosensory cortical areas of t wo rhesus monkeys performing a tactile discrimination task. The subjec ts actively stroked their fingertips over gratings that varied in groo ve width. Many cells in each location displayed average firing rates t hat incrementally reflected groove width dimensions (0.5-2.9 mm). Appr oximately 10% of cortical cells were more active to surfaces with narr ower grooves, i.e., had negative graded response functions. All thalam ic cells and similar to 50% of cortical cells with positive graded fun ctions to gratings also showed increased responsiveness to contact for ce. Some cells also varied their activity with stroke speed. Many thal amic, a few primary somatosensory cortical cells, and no secondary som atosensory cortical cells showed periodic firing patterns that reflect ed the spatial-temporal frequency of stimulation. Responses to grating s of nearly every cell with negative graded responses and the remainin g cortical cells with positive functions were independent of contact f orce and stroke velocity. The results only partially confirm predictio ns based on different models of texture perception that propose spatia l, intensive, or cross modal neural codes. Negative graded response fu nctions may require a form of spatial convergence across a cell's rece ptive field that has not previously been discussed by these models.