Visual segmentation of a heterochromatic field into center/surround components

The process of visual segmentation was studied by quantitatively estimating the apparent difference (i.e. the perceptual contrast) between two segments of a heterochromatic field. A chromatic test bar (Munsell 5R 6/6, 5BG 6/6, 5Y 6/6, or 5PB 6/6) was centered in an achromatic surround (Munsell N6) and the luminance contrast between the test bar and surround was varied between 0 and 39%. At equiluminance (0% luminance contrast) the chromatic border between center and surround was vague and perceptual contrast was minimal. When luminance contrast was increased slightly (5-15%) the border between center and surround was more distinct and perceptual contrast was almost two times greater than predicted by a model that combines spatial differences in chromaticity and luminance as orthogonal vectors. A luminance border may enhance perceptual contrast by increasing the salience of the center/surround hue difference. A vector model is consistent with perceptual contrast only at relatively high luminance contrasts, where hue differences play a relatively minor role in visual segmentation.