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

Abstract

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.

Effects of inter-food interval on the variety effect in an instrumental food-seeking task. Clarifying the role of habituation

“…In this sense, variety can enhance responding between-sessions by virtue of a mechanism that is different from habituation. It is worth noting, however, that Bouton et al (2013) also observed less responding to the grain pellet in the alternating group than in a group that received grain pellets consistently. Thus, the two pellets produced both positive and negative incentive contrast, and the average level of responding across sucrose and grain sessions was not different from that of the control animals.…”

“…Interestingly, Bouton et al (2013, Experiment 2) also isolated a second “variety effect” that was not linked to within-session habituation. When rats were given alternating sessions that contained grain or sucrose pellet reinforcers consistently, the amount of responding for sucrose during sucrose sessions exceeded that observed in a control group that only earned sucrose pellets in every session.…”

“…Habituation may contribute to the cessation of eating within a meal. Variety may slow this process for several reasons (Bouton, Todd, Miles, León, & Epstein, 2013). For example, because habituated responding can “dishabituate” after a new stimulus (e.g., a new food type) is presented, providing one type of food might dishabituate suppressed responding to another food.…”

“…However, Bouton et al (2013) recently studied the effect in an operant food-seeking task in rats. Using a method introduced by Aoyama and McSweeney (2001), they gave rats the opportunity to earn a 45-mg food pellet for every 4th lever press they performed in each of a series of daily 30-min sessions.…”

“…Most importantly, it was designed to separate variety’s immediate effects on habituation from the possibility that it also has an impact on food’s longer-term reinforcing value. As in the experiments of Bouton et al (2013), rats lever-pressed to earn food pellets over a series of 30-min sessions. Different groups received either consistent sucrose pellets or an unpredictable mixture of sucrose and grain pellets.…”