Spectral Bandwidths for the Detection of Colour within Random Colour Textures
We investigated the spectral properties of color detection mechanisms via a noise masking paradigm. Contrast detection thresholds were measured for equiluminant chromatic modulations in the (L-M,S-(L+M))-plane within a random texture. Each texture image was the spatial juxtaposition of color samples drawn from a bi-dimensional probability density function whose support was a sector in the equiluminant plane. The task consisted in the detection of an equiluminant signal of Gaussian shape (sigma = 1.25 deg) whose amplitude was the chromatic signal and whose pixel-wise chromaticity was spatially modulated by the noise. Each stimulus consisted of two textures (5 x 5 deg) arranged side-by-side and separated by a gap (width = 0.5 deg) with the same chromaticity as the uniform background. Subjects were asked to judge on which side of the central fixation point the signal was displayed via a 2AFC paradigm. Contrast thresholds were measured for four color directions and three sector widths at increasing levels of the average energy of the axial component of the noise. Results show that contrast thresholds are unaffected by the width of the noise sector, as previously found for temporally modulated stimuli (D'Zmura & Knoblauch, 1998). The results are consistent with the existence of spectrally broadband linear detection mechanisms tuned to the signal color direction and support the hypothesis of the existence of higher-order color mechanisms with sensitivities tuned to intermediate directions in the color space.