Visual detection analysed in terms of luminance and chromatic signals

THE ability to discriminate surfaces of different colour and luminance is a basic property of human vision. This has been studied by determining visual thresholds for test stimuli of one colour superimposed on a background of another colour. Stiles has suggested that the observer samples the responses to the test flash from a number of ‘mechanisms’ each of which adapts independently to the background1,2. In the simplest form of the model, the responses of the different mechanisms to the test flash are also treated independently, that is, a flash is seen if it exceeds the threshold criterion of any of the mechanisms. The spectral sensitivities of Stiles' mechanisms (to both test and background stimuli) are similar to the absorption and action spectra of the three normal types of human cone3–5. Thus, as a simple approximation, visual mechanisms correspond to individual classes of cone and they adapt and are sampled independently; this scheme would be consistent with the demonstration of adaptation occurring within monkey cones6.