Duration, response, and location: The influence of upcoming 32% sucrose on rats' licking or lever pressing for 1% liquid sucrose

The present study investigated whether rats' rates of licking or pressing a lever for 1% liquid sucrose delivered by a continuous reinforcement schedule would decrease (contrast) or increase (induction) when the upcoming period would allow access to 32% sucrose and whether such changes would be influenced by how long each substance was available. In Experiment 1, different groups of rats licked a spout or pressed a lever for 1% sucrose in the first half of the session and, in different conditions, for 1% or 32% sucrose in the second half. Across conditions, halves of the session were 3, 6, 12, or 24 min long. Upcoming 32% sucrose significantly decreased rates of licking at each duration whereas it increased rates of lever pressing except when access duration was 3 min. Experiment 2 replicated Experiment 1 with the exception that rats that licked did so from the same spout in both halves of the session and rats that pressed a lever collected the sucrose reinforcers in the different halves at different locations. In these procedures, upcoming 32% sucrose significantly increased rates of licking. Significant, but small, increases in rates of lever pressing were still observed. The present results suggest that continuous reinforcement or duration of access to sucrose are not primary determinants of whether contrast or induction is observed. Rather, they suggest that the type of behavior (licking versus pressing a lever) and the location at which the substances are collected and consumed play a large role in which effect occurs.