Perhaps the most fundamental questions in color-discrimination theory are: (1) how many different classes of photopic mechanisms mediate color discrimination and (2) what are their spectral sensitivities? Studies of photopic visual thresholds—obtained by using small foveal stimuli—have provided evidence related to these questions. The radiance required for threshold probability of seeing is a function of many variables of a physical and methodological sort. Where color discrimination is concerned, wavelength has been the physical variable of prime interest. Visual sensitivity (reciprocal radiance required for some criterion response) is not a smooth function of wavelength, but exhibits interesting inflections, particularly when threshold radiance is the response criterion. These inflections can be significantly enhanced under suitable conditions of chromatic adaptation, and have been thought by many workers to suggest that separate underlying mechanisms contribute differentially to threshold response as a function of test-flash wavelength and adapting conditions. In order to attempt quantitative inferences about these presumed underlying mechanisms from threshold data, another difficult question must be examined: (3) what is the nature of the interactions among the photopic mechanisms as they cooperate, fail to cooperate, or inhibit one another in the determination of a threshold probability of response? Various theoretical ideas and recent experimental evidence related to these three questions are also reviewed.
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