Abstract

The distinctness of a border formed between two equiluminant fields has been measured by three independent procedures whose results are in good agreement. These are: (1) matching against a luminance-dependent border; (2) direct rating; (3) resistance to perceptual fading during prolonged fixation. Border visibility depends almost entirely on the ratio of excitation of L and M cones. When this ratio is unity, even large differences in S-cone excitation are largely ineffective and the two fields seem to melt together perceptually. These relations are easily visualized in a chromaticity diagram proposed by MacLeod and Boynton¹ which plots S-cone excitation vs L/M excitation ratio at equal luminance. The strength of contours at equal luminance can be understood in terms of color opponent receptive fields which are critical for spatial vision and which receive input only from L and M cones. S-cones, which contribute little or nothing to luminance, appear to function mainly to provide a second dimension of chromatic vision. Their minimal contribution to border perception relates to their paucity in the receptor mosaic.

© 1989 Optical Society of America