Context-dependent tactile texture-sensitivity in monkey M1 and S1 cortex.

Caudal primary motor cortex (M1, area 4) is sensitive to cutaneous inputs but the extent to which the physical details of complex stimuli are encoded is not known. We investigated the sensitivity of M1 neurones (4 Macaca mulatta monkeys) to textured stimuli (smooth/rough or rough/rougher) during the performance of a texture discrimination task and, for some cells, during a No task condition (same surfaces; no response). The recordings were made from the hemisphere contralateral to the stimulated digits; the motor response (sensory decision) was made with the non stimulated arm. Most M1 cells were modulated during surface scanning in the Task, 88%, but few of these were texture-related, 24%. In contrast, 44% of M1 neurones were texture-related in the No task condition. Recordings from the neighbouring primary somatosensory cortex (S1), the potential source of texture-related signals to M1, showed that S1 neurones were significantly more likely to be texture-related during the Task, 57 vs 24%, than M1. No difference was observed in the No task condition (52 vs 44%). In these recordings, the details about surface texture were relevant for S1 but not for M1. We suggest that tactile inputs to M1 were selectively suppressed when the animals were engaged in the Task. S1 was spared these controls as the same inputs were task-relevant. Taken together we suggest that the suppressive effects are most likely exerted directly at the level of M1, possibly through the activation of a top-down gating mechanism specific to motor set/intention.