Somatosensory cortical activity in relation to arm posture: nonuniform spatial tuning.

1. Single unitary activity in primate somatosensory cortex (SI) was recorded while monkeys maintained a range of static arm postures. Unit discharge was related to parameters defining the posture of the arm by multiple linear regression techniques. 2. Two monkeys were trained to grasp a manipulandum presented at locations distributed throughout their workspace. The discharge of single units in SI was recorded for 3 s while the monkeys maintained contact with the manipulandum and the mean discharge rate over this hold time was related to the location of the hand and to the shoulder and elbow joint angles of the arm. 3. Unitary activity of 171 neurons in the proximal arm region of areas 3, 1, and 2 was recorded during the task. Of the total, 78 neurons had activity that varied with the location of the hand in space. Neuronal discharge typically varied monotonically with the target location, reaching a maximum at the borders of the work-space. The discharge rate in most of these neurons varied with both shoulder and elbow angles. 4. Discharge rate was related to the hand's location along three axes by means of a polynomial fit. In approximately half of the neurons, activity varied significantly only for displacements along a single axis in space. However, many neurons exhibited nonlinear relations between hand location along this preferred axis and discharge rate. Discharge rate did not vary for displacements of the hand in the plane perpendicular to this preferred axis (null plane). 5. In other neurons, discharge rate varied for hand displacements in a plane, i.e., along two perpendicular axes. Displacements of the hand along the axis perpendicular to this plane (null axis) did not affect the discharge rate. In only a small minority of neurons did discharge rate vary for hand displacements along all three axes in space. 6. The distribution of the sensitivity of the neural population to hand displacements along arbitrary directions in space was not uniform. On average, hand displacement along a vertical axis led to the smallest modulation of neural discharge, and displacement of the hand along the anteroposterior direction led to the largest modulation of activity.