WEATHERLY, J.N.; University of North Dakota: Changes in Rats’ Behavior as a Function of Upcoming Food Substance: Paradoxical Results

Research has shown that rats will decrease their consumption of a substance low in hedonic value when they will soon have the opportunity to consume a highly valued substance. Other research has shown that rats will increase their rate of operant responding for a substance low in hedonic value when they will soon have the opportunity to respond for a highly valued one. Interestingly, these opposite changes in behavior occur in procedures that employ the identical substances. Theoretically, it is possible that these paradoxical outcomes are the result of differences in the function of the behavior (consummatory vs. operant), the nature of the response (licking vs. lever pressing), and/or the rate that the substances are delivered (continuously vs. intermittently). The present study investigated these possibilities by having rats either lick a spout or press a lever for 1% liquid-sucrose reinforcers (i.e., both responses were operants) delivered by a random-interval schedule during the first half of a 30-min session. In control conditions, 1% sucrose was also the reinforcer in the second half of the session. In the treatment conditions, 32% sucrose served as the reinforcer in the second half. Across conditions, the rate of reinforcement varied from 480 to 60 reinforcers per hour. Results showed that upcoming 32% sucrose reinforcement increased subjects’ rates of lever pressing for 1% sucrose in the first half of the session at each rate of reinforcement. Upcoming 32% sucrose decreased rates of licking at each reinforcement rate. These results appear to rule out the possibility that the paradoxical changes in behavior are linked to the function of the behavior or to the availability of the substances. Rather, they suggest that the nature of the response is the key factor.