ABSTRACT

In volitional behavior, movement velocity can be varied arbitrarily. This chapter reviews evidence for the presence of a velocity controller in the basal ganglia (BG). The sensorimotor BG network is hypothesized to implement a movement velocity controller that sends reference signals to the brainstem position controllers to command voluntary movements. Many striatal projection neurons represent movement velocity, whereas nigral projection neurons represent instantaneous position. The striatonigral pathway implements a neural integrator that converts the velocity signals to position signals, which act as reference signals for position controllers. The striatopallidal (indirect) pathway implements damping or discharging of the integrator. Opponent BG outputs command position controllers that move parts of the body in opposite directions along a particular axis of motion. Velocity control is only one example of a broader category of transition control, defined as the control of the rate of change in perceptual representations. It is hypothesized that the BG circuits implement a variety of transition control systems.