The spatial tuning of chromatic mechanisms identified by simultaneous masking

We have investigated the spatial transfer characteristics of the mechanisms sensitive to color in the human visual system using a method of simultaneous spatial masking with isoluminant chromatic stimuli. The test stimuli were Gaussian enveloped red-green gratings of three spatial frequencies in the lowpass region of the color domain (0.25, 0.5 and 1 c/deg). The masking stimuli were red-green gratings at the orientation and phase of the test, presented at the same spatial frequency, and at +/- 1, and +/- 2 octaves from its spatial frequency. We obtained test contrast threshold as a function of mask contrast for a wide range of mask contrasts (TvC functions). Tuning functions were derived from linear fits of the masking data, by taking the mask contrast that doubled the minimum test threshold at each spatial frequency. Chromatic tuning functions show bandpass characteristics for all test spatial frequencies examined with an average full bandwidth at half-height of 2.6 octaves, which is similar to the luminance bandwidths obtained under comparable conditions. Thus, our results suggest that the color contrast sensitivity function is the upper envelope of a range of bandpass mechanisms whose peaks extend to very low spatial frequencies.