PubMed · 14866511
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1951-08-18. COLOUR vision.. https://pubmed.ncbi.nlm.nih.gov/14866511/
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We evaluated orientation discrimination in color and luminance vision using an external noise paradigm. Stimuli were spatiotemporal patches of 2D orientation noise isolating the achromatic, red-green and blue-yellow mechanisms, and matched in multiples of contrast detection threshold. We found a monotonic increase of orientation discrimination thresholds with the stimuli orientation bandwidths that is similar for both color and luminance contrasts. This dependence was fitted with two suitable models. A variance summation model suggests that internal orientation noise is significantly greater for the chromatic than for the achromatic mechanisms, while the efficiencies are similar. A gain control model of orientation tuning suggests that both chromatic and achromatic mechanisms are characterized by broadly tuned orientation detectors and that the relative chromatic deficit in orientation discrimination may only result from a slightly broader orientation tuning for the chromatic mechanisms. The moderate deficiency in chromatic orientation discrimination may account for the small differences found in shape perception between color and luminance vision.
This study provides evidence for the existence of a low-level chromatic motion mechanism and further elucidates the conditions under which its operation becomes measurable in an experimental stimulus. Observers discriminated the direction of motion of amplitude modulated (AM) gratings that were defined by luminance or chromatic variation and masked with spatiotemporally broadband luminance or chromatic noise. The size and retinal location of the stimuli were varied and the effects of broadband noise and grating masks were both compared with the cohort of stimuli. Some significant disparities in the published literature were well explained by the results. In conclusion, evidence for a chromatically sensitive motion mechanism that evades the, detrimental effects of a luminance mask was found only at the fovea and only when the stimulus was small and centrally placed.
The 'colour-shading effect' describes the phenomenon whereby chromatic variations affect the magnitude of perceived shape-from-shading in luminance patterns. A previous study showed that in mixed colour-plus-luminance sine-wave plaids, impressions of depth in the luminance component were enhanced by non-aligned chromatic components, and suppressed by aligned chromatic components [Nature Neuroscience 6 (2003) 641-644]. Here we examine the chromatic determinants of these effects. Colour contrast was defined along the cardinal axes of colour space in order to isolate the L-M and S-(L+M) post-receptoral chromatic mechanisms. We found no difference in the potency of L-M-only and S-(L+M)-only gratings, either for enhancing or suppressing perceived depth. Moreover, the magnitude of depth-suppression was no different for any combination of depth-enhancing and depth-suppressing cardinal directions. Finally we tested whether the visual system carried the assumption that natural shading is tinged with blue, by measuring perceived depth in a colour-plus-luminance grating that was made to appear either bright-yellow/dark-blue or bright-blue/dark-yellow. However there was no difference in the magnitude of depth-suppression between conditions, suggesting that the visual system does not make any assumption about the colour of natural shading. Taken together, the results suggest that while the colour-shading effect is highly sensitive to colour contrast, it is agnostic with respect to colour direction.