Detection of isoluminant stimuli and isoluminant induction

Chromatic adaptation and induction are usually described by two-process models (Ware Cowan, 1982). At the initial stage of these models, the sensitivities of the three cone types are adjusted (von Kries, 1905). According to a more recent investigation, adaptation may be explained by just these gain changes (Chichilnisky Wandell, 1995). The second stage of these models consists of mutually inhibitory interactions within the chromatic mechanisms. By use of heterochromatic flicker photometry and detection experiments a special color metric is developed: stimuli of equal brightness with an amplitude at 5-fold detection threshold to the white point in the center of cone contrast space are specified by one parameter, their azimuth. At detection threshold, the three cone contrasts and also the excitations of the chromatic mechanisms are sinusoidal functions of this azimuth. The three cone contrast functions yield zero-crossings at different angles and so do the chromatic mechanisms. Using an infield-surround stimulus this phase information allows to distinguish between gain changes at the receptors and opponent interactions within the chromatic mechanisms. S-cones are found to influence brightness and red-green detection. Binocular presentation of the infield-surround stimulus reveals that chromatic induction is mediated by interactions within the opponent chromatic mechanisms. No adaptational components are required to explain the binocular results. Dichoptic presentation reveals global gain controls at the receptor level in addition to the local opponent interactions obtained in the binocular experiments