In Pavlovian conditioning, the strength of a conditioned response is a function of the probability of reinforcement. However, manipulations of probability are often confounded with changes in the rate of reinforcement. Two between-group experiments in mice evaluated the effect of the probability of reinforcement, while controlling the rate of reinforcement, on appetitive conditioning and extinction. Experiment 1 equated the reinforcement rate by manipulating the number of reinforcements received in each reinforced trial in a critical group (one vs. two consecutive rewards). The results of this experiment showed that probability influenced the rate of responses in acquisition, even when controlling the reinforcement rate. Experiment 2 further assessed the role of probability on behavior while controlling the rate of reinforcement during the conditioned stimulus (CS) using a split-trial design, in which the total CS time was held constant but presented in different numbers of discrete trials (e.g., 50% reinforcement with two 12 s CS's vs. 100% reinforcement with a 24 s average CS duration). This experiment confirmed that probability influenced response rates, and both the probability and rate of reinforcement affected the proportion of trials with responses. Together, these results suggest that the probability of reinforcement, while having little effect on the speed at which responses emerged, affects responding even when the rate of reinforcement is held constant. The results challenge formal learning theories to account for the effects of both the probability and rate of reinforcement. (PsycInfo Database Record (c) 2024 APA, all rights reserved).
