Evidence for the contribution of S cones to the detection of flicker brightness and red-green

We were interested in the question of how cones contribute to the detection of brightness, red-green, and blue-yellow. The linear combination of cone signals contributing to flicker detection was determined by fitting a plane to 64 points (colors) of equal heterochromatic flicker brightness. A small S-cone contribution to flicker brightness of similar amplitude in all five subjects was identified. The ratio of L- to M-cone contribution was found to vary considerably among subjects (1.7-4.1). Chromatic detection threshol ds were determined for small patches in the isoluminant plane defined by fl icker brightness. These stimuli were presented at an eccentricity of 40 are min. By using color naming at the detection threshold, one can attribute d ifferent segments of the resulting detection ellipses to different chromati c mechanisms. Linear approximation of these segments provided an estimate f or the contribution of the different cone types to the detection of red-gre en and blue-yellow. The results are consistent with the hypothesis that S c ones contribute to the red-green mechanism with the same sign as that of th e contribution from L cones. The blue-yellow mechanism very probably subtra cts S-cone contrast from luminance contrast. The detection ellipse can be m apped into a circle in cone difference space. The base of this canonical tr ansformation is a set of three cone fundamentals that differs from previous ly published estimates. Projecting the circle onto the three cone differenc e axes produces sinusoidal changes within the respective excitations. We pr opose that simultaneous sinusoidal changes of equal increment in the three cone difference excitations generate stimuli differing by equal saliency. ( C) 2000 Optical Society of America [S0740-3232(00)01106-6].