The Representation of Movement Direction in the Motor Cortex: Single Cell and Population Studies

Dynamic aspects of cortical function - 1984-01-01Georgopoulos AP, Kalaska JF, Crutcher MD, Caminiti R, Massey JT
In this chapter we discuss neuronal mechanisms in the motor cortex that may underly the control of the direction of two-dimensional arm movements. In this respect, a characteristic of single cells is their broad directional tuning, reflected in a bell-shaped, often sinusoidal, directional tuning curve. The functional meaning of this curve is two-fold: First, it denotes a directional preference, and second, it suggests the participation of a cell in the control of many directions of movement - not just of the one movement made in the cell's preferred direction. Therefore, the production of a movement in a particular direction is viewed as a function of a neuronal ensemble. This ensemble is directionally heterogeneous, for the preferred directions of the constituent cells var from cell to cell. We postulate that such a neuronal population may subserve all movement directions within a part of extrapersonal space as follows. It is hypothesized that individual cells make a vectorial contribution of variable strength along the axis of their preferred direction, according to the change in discharge rate. The vector sum of these directional contributions may represent the population code for movement direction. Population vectors of various directions can be generated by varying the kind of charge (increase or decrease in activity) and the magnitude of change in the discharge rate of the constituent cells; such variation is indeed observed in individual cells, as evidenced by the directional tuning curve. Analysis of experimental data using these assumptions yielded population vectors whose direction was close to that of the movement. Moreover, a time-course analysis showed that the very first changes in population activity preceding movement were directional and predicted the direction of the upcoming movement.