Primate motor cortex and free arm movements to visual targets in three-dimensional space. II. Coding of the direction of movement by a neuronal population.

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.