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.
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