Reaching to Visual Targets: Coordinate Systems Representation in Premotor and Motor Cortices

Abstract

Individual arm-related neurons in both motor (area 4) and premotor (area 6) cortices of the monkey are directionally tuned. We studied these directional neurons while monkeys made arm movements of similar directions within different parts of 3-D space. The behavioral task was aimed at dissociating the direction of movement, which remained similar across the work space, from the pattern of muscular activity and joint rotations underlying these movements. Within a given part of space, motor and premotor cortical cells fired most for a given preferred direction and less for other directions of movement. These preferred directions covered the directional continuum in a uniform fashion across the work space. As movements of similar directions were made within different parts of the work space, the cells’ preferred directions in both motor and premotor cortices changed their orientation. Although these changes had different magnitudes for different cells, at the population level, they followed closely the changes in orientation of the arm necessary to move the hand from one part of the work space to another.

In both premotor and motor cortices, neuronal movement population vectors accurately described the direction of movement. In contrast to the individual cells, neuronal movement population vectors did not change their spatial orientation across the work space, suggesting that they remain good predictors of movement direction regardless of the region of space in which movements are made.