The activity of single cells in the motor cortex was recorded while monkeys made arm movements in eight directions (at 45' intervals) in a two-dimensional apparatus. These movements started from the same point and were of the same amplitude. The activity of 606 cells related to proximal arm movements was examined in the task, 323 of the 606 cells were active in that task and were studied in detail.The frequency of discharge of 241 of the 323 cells (74.6%) varied in an orderly fashion with the direction of movement. Discharge was most intense with movements in a preferred direction and was reduced gradually when movements were made in directions farther and farther away from the preferred one. This resulted in a bell-shaped directional tuning curve. These relations were observed for cell discharge during the reaction time, the movement time, and the period that preceded the earliest changes in the electromyographic activity (-80 msec before movement onset). In about 75% of the 241 directionally tuned cells, the frequency of discharge, D, was a sinusoidal function of the direction of movement, 8: D = by + &sin 8 + &OS 8, or, in terms of the preferred direction, 80: D = b0 + c1cos(8 -&), where bO, bl, b2, and cl are regression coefficients. Preferred directions differed for different cells so that the tuning curves partially overlapped.The orderly variation of cell discharge with the direction of movement and the fact that cells related to only one of the eight directions of movement tested were rarely observed indicate that movements in a particular direction are not subserved by motor cortical cells uniquely related to that movement. It is suggested, instead, that a movement trajectory in a desired direction might be generated by the cooperation of cells with overlapping tuning curves. The nature of this hypothetical population code for movement direction remains to be elucidated.The importance of the motor cortex for voluntary limb movements in the primate is well established. It is supported by the results of lesion, electrical stimulation, and single cell recording experiments carried out by many investigators (reviewed in detail

We describe a code by which a population of motor cortical neurons could determine uniquely the direction of reaching movements in three-dimensional space. The population consisted of 475 directionally tuned cells whose functional properties are described in the preceding paper (Schwartz et al., 1988). Each cell discharged at the highest rate with movements in its "preferred direction" and at progressively lower rates with movements in directions away from the preferred one. The neuronal population code assumes that for a particular movement direction each cell makes a vectorial contribution ("votes") with direction in the cell's preferred direction and magnitude proportional to the change in the cell's discharge rate associated with the particular direction of movement. The vector sum of these contributions is the outcome of the population code (the "neuronal population vector") and points in the direction of movement in space well before the movement begins.

We describe the relations between the neuronal activity in primate motor cortex and the direction of arm movement in three-dimensional (3-D) space. The electrical signs of discharge of 568 cells were recorded while monkeys made movements of equal amplitude from the same starting position to 8 visual targets in a reaction time task. The layout of the targets in 3-D space was such that the direction of the movement ranged over the whole 3-D directional continuum in approximately equal angular intervals. We found that the discharge rate of 475/568 (83.6%) cells varied in an orderly fashion with the direction of movement: discharge rate was highest with movements in a certain direction (the cell's "preferred direction") and decreased progressively with movements in other directions, as a function of the cosine of the angle formed by the direction of the movement and the cell's preferred direction. The preferred directions of different cells were distributed throughout 3-D space. These findings generalize to 3-D space previous results obtained in 2-D space (Georgopoulos et al., 1982) and suggest that the motor cortex is a nodal point in the construction of patterns of output signals specifying the direction of arm movement in extrapersonal space.