Crespi Effect

The Crespi effect is a behavioral contrast phenomenon observed in classical conditioning, in which a conditioned response changes by more than would be expected once the reinforcement behind it is suddenly changed. It was first observed in rats by the American psychologist Leo P. Crespi in 1942, who found that in a repeated task such as finding food in a maze, a rat's running speed tracked the size of the reward it had received on the previous trial, so that a larger reward produced faster running on the next attempt. The effect also ran in reverse: rats shifted from a larger reward down to a smaller one ran more slowly than rats that had always received only the small reward. Crespi found that the size of the reward had little or no effect on how quickly the rats learned the task in the first place, only on how they performed a task they had already learned, and later researchers have only partially been able to replicate his original findings, which remain somewhat controversial.