Behavioral contrast refers to a change in the strength of one response that occurs when the rate of reward of a second response, or of the first response under different conditions, is changed. For example, suppose that a pigeon in an operant chamber pecks a key for food reward. Sometimes the key is red, sometimes green, but food comes with equal frequency in either case. Then suddenly pecking the key when it is green brings food less frequently. Positive contrast is seen when the rate of response to the red key goes up, even though the frequency of reward in red remains unchanged. Likewise, increasing the reward to green tends to reduce the response rate to red (negative contrast). This sort of contrast effect may occur following changes in the amount, frequency, or nature of the reward, and it has been shown to occur with various experimental designs and response measures (e.g. response rate, running speed).