Color and brightness contrast effects as a function of spatial variables

The contrast effect was measured at the center of a circular center-surround stimulus display for three color combinations and one (non-colored) brightness combination. For each combination, the magnitude of the contrast effect was determined for a series of different center and surround sizes using consecutive matching procedures. The amount of contrast was influenced by the dimensions of the display in the expected way; the effect on the center was greatest when the surround was large and the center was small. The contrast magnitudes were plotted against four theoretically selected abscissae: (1) surround width, R 1 − R 2; (2) surround area, R 1 2 − R 2 2; (3) area ratio, ( R 1 2 − R 2 2)/ R 2 2; and (4) edge-distance expression 1/ R 2−1 R 1, where R 2 is the radius of the central test area and R 1 is the outer radius of the surround. For each color combination, both graphical and correlational analyses demonstrated that contrast magnitude is more closely related to the edge-distance expression than to the other spatial expressions. This result provides support for an edge-distance model based on two assumptions: (1) that edges in the stimulus, and edge detectors in the visual system, are the important determinants of color and brightness; and (2) that edges closer to a point contribute more to the determination of the color and brightness at that point than do edges that are further away.