However, even in these early experiments, the firing rates of some motor cortex cells appeared to relate to earlier planning stages. Moreover, even when a monkey was trained to withhold the movement for a certain period after the cue, the firing rates of motor cortex neurons still changed despite the absence of any movement. Such “set-related” activity has been amply confirmed in a variety of other tasks and suggests that motor cortex activity may be involved in the earlier planning stages along with activity in other motor areas of the cortex. It also suggests the possibility that other, perhaps subcortical, systems may be needed to generate a trigger signal for the initiation of movement.
In subsequent studies, researchers have used tasks in which animals were trained to move a manipulandum (a device with a handle to hold and a small circle on the end) to capture lighted targets on a surface in front of them (Fig.
9.18A ). These experiments demonstrated that cells in the arm region of the motor cortex showed changes in their firing rates in response to movement in many different directions and thus were described as broadly tuned (see
Fig.
9.18B ). That is, a cell that showed a maximal increase for movement in one particular direction, called its preferred direction, would also show somewhat smaller increases or even decreases for movement in other directions (see
Fig.
9.18C ). Moreover, the preferred directions of the different cells were uniformly distributed across all 360 degrees of possible movement directions.
These results implied that a particular cell is probably involved in most arm movements, but they also raised the issue of how precise movements could be made with such broadly tuned cells. It was suggested that although changes in the activity of individual cells could not precisely predict or specify the direction of the upcoming movement, the net activity of the population could. To test this idea, models were made in which the activity of each cell is represented as a vector (see
Fig. 9.18D ). The direction of each cell vector is determined by the preferred direction of the cell, and the magnitude of the vector for a particular movement is proportional to the firing rate of the cell during the time preceding the movement. The individual cell vectors (see
Fig. 9.18D , black lines) from hundreds of cells can then be vectorially summed to get a resultant or population vector (see
Fig. 9.18D , red lines) that accurately predicts the upcoming movement.
One of the difficulties in assessing the relationship between firing of cortical cells and various movement parameters, such as force, velocity, displacement, and target location, is that these parameters are normally correlated with each other. Therefore, variations of the tasks described earlier have been used to decorrelate these various parameters (e.g., using weights to vary the force needed to make a movement without changing the displacement, as illustrated in
Fig. 9.17A , or rotating the starting position of the wrist so that different muscles are required to generate the same trajectory in external space). The results of these experiments showed that the activity of motor cortex cells may be related to each of the various motor planning stages. Furthermore, the activity of a single cell may be correlated with one parameter initially and then switch as the time for onset of movement approaches.
Activity in the premotor and supplementary motor areas is in many ways similar to that in the primary motor cortex. Cells in these areas show activity related to upcoming