Central excitation and inhibition in conditioned reflexes

Cortical neuronal reactions were studied in cats during the conditioning of a defensive reflex to auditory stimuli. Conditioned cortical neuronal reactions were polyphasic, in which the initial responses, the temporal depression of activity, and the early and late afterdischarges could be distinguished. In many neurons during conditioning, a strengthening of initial impulse responses was found, as was the appearance and augmentation of afterdischarges. Comparison between the latency of conditioned movements and phases of neuronal reactions suggested that the conditioned reactions represented primarily the modified afterdischarges evoked by conditioned stimuli. Elimination of the reinforcement was accompanied by the gradual recovery of neuronal reactions to the initial level. Early afterdischarges were most stable during extinction. In contrast to usual extinction, differentiated inhibition was found to be a more active process: in some neurons ordinary extinction occurred, while in the others the impulse reactions arose and were augmented. It is concluded that changes of excitatory processes underlie central inhibition.