Negative and Positive Behavioural Contrast in the Same Animals

Abstract

THE reactions of animals to situations that suddenly become more attractive or less attractive have long seemed potentially useful for the evaluation of drugs and other treatments assumed to affect emotional responsivity1. Crespi2,3 reported that rats shifted to a larger (or smaller) amount of reward ran faster (or slower) to the goal in a runway than control rats who were accustomed to the larger (or smaller) amount. But it has usually been difficult to find reliable overshooting or “positive contrast” effects to an increase in reward magnitude, and even the more reliable negative contrast effect (undershooting to a decrease in reward magnitude) has proved troublesome when separate experimental and control groups are used4. Furthermore, the transient character of the effects, even when obtained, makes them inconvenient for the chronic testing of various treatments, such as antidepressant drugs or brain lesions. Finally, recent demonstrations of “behavioural contrast” have depended on somewhat complex multiple schedule situations, not necessarily related to that originally used by Crespi5–7. Here we report a simple technique for the repeated study of both positive and negative effects in the same animals, with results that are stable over weeks of daily testing.