There has been considerable recent controversy over the empirical form of the function relating response rate to reinforcement rate on Variable Interval (VI) schedules. Some theories (matching, for example) predict a monotonic relationship between response rate and reinforcement rate. Other theories (behavioral economics, for example) predict a bitonic relationship. There is empirical support for both positions. Recently, Dougan, Kuh, and Vink (in press) have shown that session length is one variable which alters the form of the function. Functions were predominantly monotonic when sessions were short (10 minutes), and predominantly bitonic when sessions were long (30 minutes), when data from the entire session were considered. However, the degree of bitonicity increased in successive 10-minute blocks within the 30-minute session, which is consistent with satiation processes. The present experiments further examined the effects of session length and satiation on the VI function. In Experiment 1, 8 rats were exposed to a ten-minute session either immediately preceding or immediately following a 20-minute time-out. Placement of the session relative to the time-out period had no effect on the form of the response function. In Experiment 2, 7 rats were exposed to a 10-minute session followed by a 20-minute time-out period. On some occasions, animals were prefed before the session. The response function was different depending on the prefeeding condition. The results are not consistent with recent demonstrations by McSweeney and her colleagues that response rates change systematically within reinforcement sessions. The results also suggest that satiation may play a role in the form of the VI response function.



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