Abstract

Animals and humans have a wide variety of motor repertoires, and for this reason they have to choose one motor action among many others. The mechanisms in the basal ganglia seem ideal for this purpose. The basal ganglia normally keep inhibiting their target structures, including the thalamocortical networks and the subcortical motor netwoks. This tonic inhibition can be removed by direct inhibitory inputs from the striatum carrying specific sensorimotor signals, leading to the execution of a particular movement, or can be enhanced by indirect inputs from the striatum carrying more diverse signals, leading to the suppression of unwanted movements. An important question is when this selection mechanism is deployed. Suppose you choose an action A, but not B. It may be because first, you can obtain more rewards by doing A than B, and second, you would be punished by doing B. Many recent studies have been aimed at understanding the first type of selection, reward-obtaining actions. Experiments on monkeys and humans performing goal-directed behaviors have shown that sensorimotor signals carried by striatal neurons are strongly modulated by the expected value of rewards. Such reward-dependent modulation may be caused by inputs from dopamine neurons located in the substantia nigra and surrounding areas. Importantly, the dopamine signal represents reward prediction errors which would guide the cortico-basal ganglia network to choose the optimal output for obtaining rewards. In contrast, the mechanisms for punishment-avoiding behaviors are less well known. If a punishment is predicted after an action, the indirect mechanism in the basal ganglia may become active to suppress the action. This enhancement of the indirect mechanism may be caused by a punishment-predictive signal which originates from the lateral habenula and is mediated by dopamine neurons.