SOMATOSENSORY CORTICAL ACTIVITY IN RELATION TO ARM POSTURE - NONUNIFORM SPATIAL TUNING

1. Single unitary activity in primate somatosensory cortex (SI) was re corded while monkeys maintained a range of static arm postures. Unit d ischarge 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 the ir workspace. The discharge of single units in SI was recorded for 3 a while the monkeys maintained contact with the manipulandum and the me an discharge rate over this hold time was related to the location of t he hand and to the shoulder and elbow joint angles of the arm. 3. Unit ary 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 activ ity that varied with the location of the hand in space. Neuronal disch arge typically varied monotonically with the target location, reaching a maximum at the borders of the workspace. The discharge rate in most of these neurons varied with both shoulder and elbow angles. 4. Disch arge rate was related to the hand's location along three axes by means of a polynomial fit. In approximately half of the neurons, activity v aried significantly only for displacements along a single axis in spac e. However, many neurons exhibited nonlinear relations between band lo cation along this preferred axis and discharge rate. Discharge rare di d 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 perpendicul ar axes. Displacements of the hand along the axis perpendicular to thi s plane (null axis) did not affect the discharge rate. In only a small minority of neurons did discharge rate vary for hand displacements al ong all three axes in space. 6. The distribution of the sensitivity of the neural population to hand displacements along arbitrary direction s in space was not uniform. On average, hand displacement along a vert ical axis led to the smallest modulation of neural discharge, and disp lacement of the hand along the anteroposterior direction led to the la rgest modulation of activity.