Chapter 13 Motor cortex: Neural and computational studies

The motor cortex can be regarded as a network of neurons processing, inter alia, spatial motor information. A basic component of this information is the direction of movement in space. Experimental studies in behaving monkeys have shown that the impulse activity of single motor-cortical cells relates to this component in an orderly fashion, such that the frequency of cell discharge is a sinusoidal function of the direction of movement, with the direction for which cell discharge is highest denoting the "preferred direction" of the cell. The neuronal ensemble of such directionally tuned cells can be regarded as a network in which each cell is represented as a vector pointing in the cell's preferred direction. The network operates to generate a signal in the direction of a desired movement. We regard this operation as the vectorial summation of the cell vectors, weighted by a scalar measure of the intensity of cell activation. The resulting vector sum is called the "neuronal population vector." Analysis of experimental data has shown that the population vector points in the direction of the movement. In addition, the population vector can be calculated as a time-varying signal and, as such, is a robust predictor of the direction of the upcoming movement during the reaction time and during instructed and memorized delays. Finally, it has proven a good tool for monitoring and deciphering directional information when more complex directional operations are performed.