How surrounds affect chromaticity discrimination.

Chromatic discrimination thresholds were measured with and without surrounds along two cardinal axes of chromaticity space. On one axis the level of short-wavelength-sensitive (SWS)-cone excitation was varied for constant long-wavelength-sensitive (LWS)-cone and medium-wavelength-sensitive (MWS)-cone excitations, and on the other axis there were equal and opposite changes in LWS-cone and MWS-cone excitations for constant levels of SWS-cone excitation. Results for two of three observers showed that with a dark surround, discrimination mediated by SWS cones was regulated by the level of SWS-cone excitation of the starting chromaticity, showing a function with the form of a threshold-versus-radiance function. For an equiluminant white or yellow surround, the discrimination for all three observers showed a minimum at the level of SWS-cone excitation of the surround, giving a V-shaped function for the white surround. An additional experiment with dimmer white surrounds indicated that while the minimum remained at the white point, the function gradually changed toward the shape with a dark surround. Discrimination thresholds mediated by LWS and MWS cones with a dark surround showed a minimum near the LWS-cone excitation of equal-energy white, giving a V-shaped function. The effect of yellow and white surrounds was to deepen the V. The data can be described by a model of chromatic discrimination incorporating a threshold term, a cone gain control, and an opponent gain control into two equations, one for SWS-cone discrimination and one for LWS-cone and MWS-cone discrimination.