Behavioral Contrast

Behavioral contrast is a change in the strength of one learned response that occurs when the rate of reward for a second response, or for the same response under different conditions, is changed. Leo Crespi first studied it systematically in 1942, training rats to run toward a food reward: rats shifted from a large reward to a small one ran more slowly than rats trained on the small reward from the start, a pattern called negative contrast, and the reverse pattern, running faster than expected after being shifted to a larger reward, is called positive contrast. In experiments offering two response options, such as a pigeon that can peck at a red key or a green key, cutting the reward available for pecking the green key can raise the rate of pecking at the red key even though the red key's own reward has not changed. The effect shows that behavior responds to how a reward compares with other available rewards, not only to the size of that reward on its own, and it shows up across different kinds of rewards, including their amount, frequency, and nature.