Activity of substantia nigra neurons in the cat brain during a self-initiated behavioral act

Cats were trained to perform a self-initiated behavioral act in the form of an operant food-obtaining reflex with defined time requirements. Activity was recorded from 50 dopaminergic neurons (identified in terms of their low frequency of background activity and long action potentials) and 67 nondopaminergic neurons of the substantia nigra and adjacent region. Dopaminergic neurons were the more responsive. Prior to EMG activation, the activity of 33 (66%) of these cells changed, and 44 (88%) showed changes in activity on movement. Dopaminergic neurons showed increased activity during the period of waiting for the conditioned stimulus, predicting the release of reinforcement or its absence. These cells were more frequently activated in response to a positive signal and reinforcement and were more frequently inhibited in the absence of reinforcement. The high reactivity of dopaminergic neurons during execution of a movement task could be explained by the involvement of a cognitive component, i.e., determining the point at which the movement should start.