Correlation of fingertip shear force direction with somatosensory cortical activity in monkey

To examine the activity of somatosensory cortex (S1) neurons to self-generated shear forces on the index and thumb, two monkeys were trained to grasp a stationary metal tab with a key grip and exert forces without the fingers slipping in one of four orthogonal directions for 1 s. A majority (∼85%) of slowly adapting and rapidly adapting (RA) S1 neurons had activity modulated with shear force direction. The cells were recorded mainly in areas 1 and 2 of the S1, although some area 3b neurons also responded to shear direction or magnitude. The preferred shear vectors were distributed in every direction, with tuning arcs varying from 50° to 170°. Some RA neurons sensitive to dynamic shear force direction also responded to static shear force but within a narrower range, suggesting that the direction of the shear force may influence the adaptation rate. Other neurons were modulated with shear forces in diametrically opposite directions. The directional sensitivity of S1 cortical neurons is consistent with recordings from cutaneous afferents showing that shear direction, even without slip, is a powerful stimulus to S1 neurons.