RECEPTOR NOISE AS A DETERMINANT OF COLOR THRESHOLDS

Inferences about mechanisms at one particular stage of a visual pathwa y may be made from psychophysical thresholds only if the noise at the stage in question dominates that in the others. Spectral sensitivities , measured under bright conditions, for di-, tri-, and tetrachromatic eyes from a range of animals can be modelled by assuming that threshol ds are set by colour opponency mechanisms whose performance is limited by photoreceptor noise, the achromatic signal being disregarded, Nois e in the opponency channels themselves is therefore not statistically independent, and it is not possible to infer anything more about the c hannels from psychophysical thresholds. As well as giving insight into mechanisms of vision, the model predicts the performance of colour vi sion in animals where physiological and anatomical data on the eye are available, but there are no direct measurements of perceptual thresho lds. The model, therefore, is widely applicable to comparative studies of eye design and visual ecology.