Post-reinforcement electrocortical synchronization and facilitation of cortical auditory evoked potentials in appetitive instrumental conditioning

Abstract

Food deprived cats trained to press a lever on a variable interval schedule for milk reward displayed bursts of relatively large amplitude spindle ECoG activity topographically restricted to the parieto-occipital region during the consumption of reward. An attempt was made to explore the mechanisms involved in these ECoG phenomena known as post-reinforcement synchronization (PRS). When each lever press was rewarded, the parieto-occipital cortex displayed almost continuous spindle bursts whereas the somato-sensory cortex remained desyncronized. In trained subjects both milk or water produced identical PRS. However, when water was substituted for milk during the course of an experiment, PRS was completely suppressed and overt signs of emotionality appeared. Subjects allowed to drink milk from a bowl displayed a similar topographically restricted ECoG synchronization. Both types of consummatory ECoG synchronization were suppressed in darkness or by novel environmental stimuli, and both were indiscernible from sleep-spindle ECoG activity recorded from the same region in satiated subjects, although during sleep the occurrence of spindles did not depend on the presence of light and their cortical distribution was more diffuse. Subjects were habituated to a tone (850 c/sec) or click which did not disrupt their performance and the occurrence of PRS or sleep. The primary and secondary evoked potentials recorded from various cortical projections during bursts of PRS were conspicuously augmented. Their amplitude, much greater than that recorded during relaxed wakefulness in satiated subjects, was comparable to that observed during spindle-slow wave sleep. The evoked potentials recorded after non-rewarded lever pressing; i.e., during desynchronized ECoG activity were poorly developed; they were comparable to those observed during emotional excitement. Considering the similarity of PRS patterns and sleep ECoG synchronization and assuming that the amplitude of auditory evoked potentials produced by an irrelevant stimulus reflects the fluctuations in the tonus of the reticular activating system (RAS), our results suggest that: (1) PRS is triggered by a transient depression of RAS; and (2) there is a functional overlap between reward centers and hypnogenic ( i.e., synchronizing) influences.