Abstract
Purpose: :
Color discrimination is conventionally thought to depend on two neural signals. One signal, carried by the midget ganglion cells, represents the ratio of the long- (L) and middle-wave (M) cone excitations; it corresponds to the horizontal axis of the MacLeod-Boynton chromaticity diagram. The second signal, carried by the small bistratified ganglion cells, represents the ratio of short-wave (S) cone excitation to some combination of L and M excitations; it corresponds to the vertical axis of the MacLeod-Boynton diagram. However, a line from unique blue through white to unique yellow runs obliquely in the diagram. Our purpose was to measure discrimination across this line, i.e. discrimination between greenish and reddish hues.
Methods: :
Observers were asked to discriminate between the two halves of a circular 2-deg field divided vertically by a thin line. Each half-field was independently jittered in luminance to ensure that discrimination was based on chromaticity. Target duration was 150 msec. The background was metameric to Illuminant D65. The observer was asked to make a spatial forced-choice, indicating which hemifield was the greener. Feedback was given by auditory signals. To choose our stimuli, we rescaled the vertical axis of the MacLeod-Boynton diagram so that a line passing through unique yellow (576 nm) and the metamer of Illuminant D65 had a slope of -45 deg. We then selected a series of lines that intersected this line at 90 deg, and we measured discrimination along these lines. In a given block of trials, the chromaticities were always equidistant from a given point on the test line but their separation was increased or decreased according to the subject’s performance. In different blocks, we probed different positions along the test line. Thresholds were expressed in terms of the factor by which L/(L+M) was changed.
Results: :
Discrimination thresholds exhibit a marked minimum close to the transition between reddish and greenish hues (a transition we independently assessed by interleaved phenomenological measurements). These minima are not aligned with either of the axes of the MacLeod-Boynton diagram.
Conclusions: :
The chromaticities at which discrimination is optimal may be those at which a neural channel is in its equilibrium state. Our psychophysical results suggest a chromatic channel that draws signals of the same sign from S and L cones and an opposed signal from M cones.
Keywords: color vision • perception