Definitions of color channels

Before reviewing the psychophysical evidence for chromatic input to motion processing, it is necessary to review briefly the different dimensions of color vision. Theories of color vision posit the existence of three postreceptoral channels, which are derived from the sums and differences of the three cone types in the eye. The achromatic or luminance channel signals a weighted sum of long-wavelength-selective (L) and medium-wavelength-selective (M) cones (i.e., L + M, with some debate regarding the contribution of short-wavelength-selective (S) cones; e.g., Boynton et al., 1985; Eisner and Macleod, 1980; Stockman et al., 1991). Two chromatic channels signal weighted sums and differences of the cones. The red/green chromatic channel signals differences between L and M cones (i.e., L − M or M − L). The blue/yellow chromatic channel signals differences between S-cones and the sum of L and M cones (i.e., S(L + M)). These three channels are referred to as cardinal channels, and the stimuli that isolate them are referred to as the cardinal axes of three-dimensional color space (Fig. 82.1). Note that stimuli varying along either the red/green or blue/yellow axis (or any axis within the plane of the two) do not vary in luminance; for this reason, stimuli modulated within this plane are referred to as equiluminant.

Figure 82.1..  

Three color channels. The luminance channel signals a weighted sum of long-wavelength-selective (L) and medium-wavelength-selective (M) cones, that is, L + M. The red/green chromatic channel signals differences between L and M cones, that is, L − M. The blue/yellow chromatic channel signals differences between short-wavelength-selective (S) cones and the sum of L and M cones, that is, S − (L + M). (Reprinted from Neuron, vol. 25, Dobkins, K. R., Moving Colors in the Lime Light, Pages 15–18, Copyright 2000 with permission from Elsevier Science.) (See color plate 59.)