CONTRAST. A term employed in psy chology to denote an enhancement of a sensory experience which is induced by another sensory experience of the same modality. Contrast is alleged by some psychologists to occur also in feeling, and the is occasionally loosely employed as a principle of explanation of cer tain spatial perceptions.
The contrast-effect may be successive or simultaneous. In successive contrast the in ducing precedes the induced sensation in time. For example, an orange is more sour after dessert than before, the odor of Bordeaux is stronger following the scent of cheese, a surf of constant temperature seems warm on a cold day and cold on a warm day, a room of medium illumination seems light on emerging from a dark room, and dark on entering from the bright out-of-doors. In simultaneous contrast, on the other hand, the effect is induced simul taneously and it is reciprocal with the inducing sensation. If, for instance, a sub-liminal sweet is placed on one side and a supra-liminal salt on the other side of the tongue, then the sweet becomes liminal and the salt is enhanced. The best known instances of simultaneous contrast occur, however, in the field of vision. If from the same sheet of gray paper two narrow strips are cut and placed the one on a black, the other on a white background, then the former will appear much lighter and the latter much darker than the gray sheet (brightness-con trast). If, again, the same gray strips are placed the one on a well-saturated blue, the other on a good red background, and a sheet of tissue paper laid over all, then the former strip will be seen as yellow of poor chroma, the latter as a washed-out green (color-contrast). Vivid effects of color-contrast may also be seen in shadows, as, for example, the purple or violet shadows on die sun-lit snow. The results the experimental investigation of color-contrast have been formulated in the following laws: (1) The contrast-effect is always in the direc tion of the complementary color; blue induces yellow, red induces blue-green, black induces white. (2) Color-contrast is greatest when brightness-contrast has been eliminated. (3)
The nearer together the contrasting surfaces, the greater is the contrast-effect. (4) The elimination of contours enhances the contrast effect. (5) The more saturated the color, the greater is the contrast-effect.
Two types of theory have been offered in explanation of contrast, the one physiological, the other °psychological)) Physiologically re garded, it may be said in general that successive contrast is the result of adaptation to the induc ing stimulus; an orange is more sour after dessert because the taste bulbs having become' adapted to sweet respond without compensation to sour. In smell, the odors which contrast are antagonistic so that adaptation to the one leaves the other as it were in bolder relief. In tem perature, adaptation to an aerial cold seems to have correlated with it a atunine of the sense organs of warmth (in the area concerned) so that they respond to a stimulus of lower tem perature; in the same way cold spots are °tuned) to a higher temperature of stimulus with adaptation to warmth. In vision, adaptation together with the negative after-image is suffi cient' to explain most cases of successive con trast. Adaptation fails, however, to explain cases of simultaneous contrast ; the effect of adapta tion always requires an interval of time, whereas simultaneous contrast is immediate.. The case of the sub-liminal sweet noted above may ultimately be explained by the fact that the sensory nerves connected with the taste bulbs radiate to the two sides of the tongue; in the present state of our knowledge we can only guess that closely related areas of the cortex are thereby stimulated, and that attention to the sour facilitates the simultaneous arousal of the sweet. Simultaneous brightness and color contrast has been most satisfactorily explained by Hering. This author supposes that the stimulation of a portion of the retina which in volves an assimilation (or dissimilation) of a photo-chemical substance carries with it a simultaneous dissimilation (or assimilation) of the same substance in the contiguous unstimu lated portion of the retina.