Abstract

There are two competing theories of how organisms evaluate temporally-extended appetitive experiences. The "peak" model states that the most rewarding moment of the experience is recorded into memory, while the "peak-and-end" model asserts that remembered reward value is an averaging of both the "peak" reward and the reward value experienced at the end of the event. Both models also suggest that information about the temporal duration of the experience plays little role in retrospective evaluations ("duration neglect"). The current brain stimulation reward (BSR) studies: (a) tested a mathematical model designed to predict how lever-pressing performance in self-stimulating rats is affected by varying the train duration and pulse frequency, (b) examined how train duration and pulse frequency affect "duration neglect", and (c) compared the "peak" and "peak-and-end" models. Two male rats were chronically implanted with stimulating electrodes aimed at the lateral hypothalamic area. In the first experiment, train duration was held constant while the pulse frequency was varied. In the second experiment, frequency was held constant while the train duration was varied. In the third experiment, constant-frequency trains were compared to trains in which the frequency at the "end" of the train was less rewarding than that of the "peak". The results were consistent with the mathematical model, and allowed for a better understanding of "duration neglect" in BSR. Furthermore, while one subject displayed a lack of a peak-and-end effect, the other rat's results appeared to support the peak-and-end model, although instabilities in performance in the latter rat may have led to a spurious result.