P300- like event related potential amplitude in rats is a correlate of conditioned reinforcement

We have developed a methodology for recording a robust P300 event related potential (ERP) in rats. In these experiments a contingency shaped model of the human “oddball’ paradigm was employed in which rats were shaped to press a lever for food reinforcement signaled by the click of the pellet dispenser. A target tone cued the insertion of the lever that retracted after 1-sec or immediately following a single reinforced response, while a non-target tone was randomly presented. Brain activity was recorded through stainless steel electrodes implanted 1mm below the skull. Here, we compared the amplitude of the P300 response to the click of the pellet dispenser to the amplitude of the P300 response to the target and non-target tones. We found that the amplitude to food click was significantly greater that the amplitude to the target tone that cued lever insertion. Since the food click is more proximal to the primary reinforcer than the lever tone, it is a stronger conditioned reinforcer than the lever tone that sets the occasion for the food click. Accordingly we suggest that the P300 in rats is a correlate of conditioned reinforcement.