Primate motor cortex and free arm movements to visual targets in three- dimensional space. I. Relations between single cell discharge and direction of movement

TLDR

The study investigates how primate motor cortex neuron firing relates to arm movement direction in 3‑D space. Neuronal discharge from 568 cells was recorded as monkeys performed equal‑amplitude arm movements from a common start point to eight visual targets spanning the full 3‑D directional continuum in a reaction‑time task. Most recorded cells (83.6%) showed direction‑dependent firing, peaking at a preferred direction and decreasing with the cosine of the angle to that direction, with preferred directions uniformly distributed across 3‑D space, indicating the motor cortex orchestrates arm‑movement direction signals in extrapersonal space.

Abstract

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.