The Constancy of Equiluminant Red-Green Thresholds Examined in Two Color Spaces

Abstract

Chromatic detection by the L or long-wavelength cones and M or middle-wavelength cones can be conveniently represented in the diagram shown in Fig. 1. The vertical axis represents the change in the quantal catch of the M cones, produced by a test light modulation, divided by the quantal catch in the M cones produced by all of the steady field components. The horizontal axis represents the analogous quantity for the L cones. Fig. 2 shows the simple model which justifies this representation. The cone classes adapt independently of one another before being combined into the red-green chromatic and luminance mechanisms. The model requires that adaptation take place within each cone pathway, but not necessarily in the cones themselves. Adaptation also occurs at the opponent stage. The linear combinations of cone contrasts shown in the model of Fig. 2 imply that detection contours in cone contrast space should be four straight line segments (only three of which are shown in Fig. 1). Each vector in Fig. 1 represents the threshold for a particular ratio of red to green light modulations, corresponding to a particular ratio of L-to-M cone modulations. The 45° vector represents a test stimulus with the same chromaticity as the adapting field. If there is a detection mechanism which combines cone contrasts linearly and has a response which is independent of luminance modulation, it must have a detection contour which is parallel to the 45° line - that is, it must have a slope of +1.0 — regardless of the adapting chromaticity. No such simple constraint governs the slope of the luminance detection contour. The 45°-225° direction will be here referred to as the “equichromatic” direction.