Within- and between-session variety effects in a food-seeking habituation paradigm

Appetitive behavior is stronger when organisms are given a variety of foods than when they are repeatedly given the same food (the variety effect). Two experiments examined the variety effect in an operant food-seeking task. In both experiments, rats received a 45-mg food pellet for every 4th lever press over a series of daily 30-min sessions. The rats responded at a high rate early in the session, but the rate declined systematically over time within the session. In Experiment 1, alternating unpredictably between grain and sucrose pellets caused a higher level of responding, and a slower within-session decline in responding, than presenting either type of pellet consistently. In groups receiving one pellet consistently, a switch to the alternate pellet caused lawful changes in response rate that reflected both habituation and incentive contrast processes. In Experiment 2, an experimental group received grain only and sucrose only in daily alternating sessions. In sucrose sessions, they responded more than controls that always received either sucrose or grain (a type of variety effect); in grain sessions, they responded less than the controls. The results indicated a within-session variety effect that was controlled by habituation processes and a between-session variety effect that was controlled by incentive contrast. Both types of processes can come into play when organisms are exposed to food variety.