Luminance modulates color detection threshold in natural scenes

Luminance and chrominance information is extracted from natural scenes by different cone-opponent mechanisms at early stage of visual processing. How independently the two streams are then processed along the visual pathway is still a controversial issue. While some authors believe that luminance and color signals are processed separately and in parallel until high perceptual level beyond V1 (Livingstone and Hubel, 1987), others suggest that they could interact as early as the LGN (Parraga et al., 2002). Supporting the latter assumption, Parraga et al. (2002) introduced a model which includes a divisive normalization of the color-opponent channels by luminance. Here we try to access the relevance of this model when viewing natural scenes. Using a 2AFC paradigm where subjects were asked to decide between two images of the same scene containing identical luminance information which one was colored, we found that color detection threshold increased with global luminance level. While it suggests an early interaction between chrominance and luminance channels, a misalignment of individual cone-opponent channels with our color space cardinal directions combined with a contrast normalization mechanism occurring independently in each channel could also account for our results. This explanation was discarded by measuring in a second experiment detection thresholds for luminance in the same natural scenes which under this hypothesis could have been predicted from our first results. Our findings suggest an early interaction of luminance and chrominance in the visual system and provide another argument for a nonlinear divisive normalization of color channels by luminance in natural scenes. Teaser (50 words) The visual system extract from natural scenes luminance and chrominance information but how independently these two streams are processed is still a controversial issue. Measuring the effect of luminance on color detection thresholds in natural scenes, we found an argument for a nonlinear divisive normalization of color channels by luminance.