Neuronal responses in ventroposterolateral nucleus of thalamus in monkeys (Macaca mulatta) during active touch of gratings.

Responses of single neurons were recorded from the ventroposterolateral nucleus (VPL) of the thalamus while a monkey stroked its fingertips over gratings. Monkeys were trained to stroke the gratings with consistent downward applied force and velocity of hand motion. Neurons were selected with receptive fields on the glabrous digits. Average firing rate was computed for a range of grating groove widths; groove width corresponded to roughness. Force and velocity were measured. VPL responses were compared to previously reported responses in primary somatosensory cortex (SI) under identical stimulus conditions, and to reports of peripheral afferent fiber responses to passively applied gratings. VPL responses more closely resembled those of peripheral afferent fibers than those of SI in important respects: lack of independent responses to roughness, force, and velocity; high temporal and force fidelity; and response patterns that closely followed the shape of elevated metal strips used to separate pairs of gratings. The presence in cortex of response patterns not seen in the thalamus, such as response independence and negative correlations to groove width, suggests that they stem from cortical processing.