Detection thresholds were obtained for a 2 degrees Gaussian-blurred spot flashed for 200 ms on an 8.9 degrees white adapting field of 1070 trolands. The spot's contrast was represented in an L-, M-, and S-cone contrast space. Detection thresholds were obtained for many vectors close to specific but theoretically important planes within this space. A three-dimensional surface was fitted to the data generated by the probability summation of three mechanisms, each a weighted sum of cone contrasts. The fit revealed a red-green chromatic mechanism driven by delta L/L--delta M/M with no S-cone input that was 1 order of magnitude more sensitive than the two other mechanisms. The latter consisted of a luminance mechanism with little S-cone input and a blue-yellow chromatic mechanism with the S cone opposed to L and M cones.
The visibility of a 1 degree, 200-msec flash on a large yellow field was measured as a function of the intensity of a coincident pedestal flash (a flash that was the same in both temporal intervals of a two-alternative forced-choice trial). The various flashes were incremental (+Lum) or decremental (-Lum) yellow luminance flashes or green (+Chr) or red (-Chr) isoluminant chromatic flashes. With uncrossed conditions (Lum tests on Lum pedestals or Chr tests on Chr pedestals), we obtained the conventional dipper function, that is, the function of threshold test intensity was highly asymmetric about zero pedestal intensity, and strong pedestals induced strong masking. Crossed conditions produced neither effect: for example, with Chr tests on Lum pedestals, there was no dipper function: the function of threshold test intensity was symmetric about zero pedestal intensity, and strong pedestals produced no masking. Instead, the suprathreshold luminance pedestals facilitated chromatic detection by as much as 2-3X and also linearized the chromatic psychometric function, further enhancing sensitivity to weak chromatic stimuli. (Chromatic sensitivity on the suprathreshold luminance pedestal was approximately 25X higher than luminance sensitivity on the uniform field.) A pedestal consisting of a thin luminance ring that surrounded the chromatic test produced facilitation equal to that of the uniform-luminance pedestal: the pedestal may thus act to demarcate the test spatially and promote chromatic comparison with the surround. Removing the uniform yellow surround eliminated this crossed facilitation but did not eliminate the uncrossed facilitation (the dipper function), suggesting that different mechanisms mediate the crossed and uncrossed facilitations.
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