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The relative difference in intensity between two stimuli and their effect on each other.

Contrast, or contrast effect, is the effect a visual stimulus has on another. When one stimulus is present, it affects the other. As can be seen in this illustration, two gray boxes of equal intensity are surrounded by, in one case, a white field, and in the other, a black field. The perceived shade of gray is affected by the contrasting field.

Psychologists also study the contrast threshold, the point at which differences in two stimuli can be detected. These tests are used in the study of visual perception and the ability to perceive spatial relationships. Understanding contrast effect has practical applications. For example, black and yellow have the lowest contrast effect, which means the largest percentage of the population can clearly detect the difference between these two colors. Therefore, black and yellow are the colors used to mark school buses and many traffic signs.

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Psychology EncyclopediaPsychological Dictionary: Abacus to Courage