General application of Contrast effect
Perception example: A neutral gray target will appear lighter or darker than it does in isolation when immediately preceded by, or simultaneously compared to, respectively, a dark gray or light gray target.
Cognition example: A person will appear more or less attractive than that person does in isolation when immediately preceded by, or simultaneously compared to, respectively, a less or more attractive person.
Performance example: A laboratory rat will work faster, or slower, during a stimulus predicting a given amount of reward when that stimulus and reward are immediately preceded by, or alternated with, respectively, different stimuli associated with either a lesser or greater amount of reward.
The contrast effect was noted by the 17th century philosopher John Locke, who observed that lukewarm water can feel hot or cold, depending on whether the hand touching it was previously in hot or cold water. In the early 20th century, Wilhelm Wundt identified contrast as a fundamental principle of perception, and since then the effect has been confirmed in many different areas. Contrast effects can shape not only visual qualities like color and brightness, but other kinds of perception, including the perception of weight. One experiment found that thinking of the name "Hitler" led to subjects rating a person as more friendly. Whether a piece of music is perceived as good or bad can depend on whether the music heard before it was unpleasant or pleasant. For the effect to work, the objects being compared need to be similar to each other: a television reporter can seem to shrink when interviewing a tall basketball player, but not when standing next to a tall building.