For example, suppose that a pigeon in an operant chamber pecks a key for food reward.
Likewise, increasing the reward to green tends to reduce the response rate to red (negative contrast).
[1][2] In 1942, Crespi measured the speed of rats running to various amounts of reward at the end of an alley.
In 1981, Bower discovered that positive contrast may be reduced because the response measure hits a ceiling.
Thus, if contrast is the subject of an experiment, reward sizes may need to be adjusted to keep the response below such a ceiling.