Post-reinforcement electrocortical synchronization and facilitation of cortical somato-sensory evoked potentials during instrumentally conditioned appetitive behavior in the cat

Abstract

Five cats bearing chronically implanted cortical electrodes were trained to press a lever for 0.5 ml of milk reward. They displayed characteristic bursts of high voltage slow wave (6–9 c/sec) activity; i.e., post-reinforcement synchronization (PRS) over the visual cortex (posterior marginal gyrus) after the presentation and during consumption of milk reward. Computer averaged somato-sensory evoked potentials (EPs) induced by mild electrical stimuli delivered to the skin were recorded over the specific and non-specific cortical projections during the following behavioral states: (1) after non-rewarded lever pressing (NOR); (2) after presentation of reward in the illuminated environment; i.e., during bursts of PRS; (3) during consumption of reward in the dark (RD); i.e., during complete suppression of PRS activity; (4) in a waking but relaxed subject after satiation (REL); (5) during emotional excitement (EXC); and (6) during the early stage of spindle slow wave sleep (SL). The subjects were habituated to mild skin stimuli whose intensity was kept 25% below the threshold which interfered with the occurrence of PRS. These stimuli were neither disrupting the subject's performance nor physiological sleep. The irrelevant character of the stimuli was maintained by their presenting at random during aforementioned six behavioral states. Most of the fourteen EPs (comprising 9 primary and 14 secondary components) recorded over frontal and parieto-occipital cortex were conspicuously augmented during bursts of PRS and during SL as compared to the corresponding EPs obtained during NOR, EXC, REL and RD. The primary responses in all PRS EPs and in six SL EPs were larger than those obtained during NOR or EXC; they were also larger in six PRS EPs as compared to those recorded during SL (two of them showed no difference and one PRS EPs was smaller); five PRS EPs were larger than those obtained during REL. All fourteen secondary responses showed a conspicuous augmentation during SL, PRS and REL as compared to the corresponding responses obtained during NOR and EXC. The amplitude of eight PRS EPs was comparable to that obtained during SL (the remaining 6 were smaller). All except one PRS EPs were larger than REL EPs (one showed no difference). Twelve PRS EPs were significantly larger than the corresponding ones recorded during RD suggesting that the visual input may play a role in the reward-induced facilitation of long latency somatosensory cortical EPs recorded over primary and secondary visual projections, and even beyond the classical visual cortex over the anterior marginal, and anterior and posterior sigmoid gyri. A possible physiological significance of the phasic facilitation of “irrelevant” sensory input caused by positive reinforcement has been discussed.