Color information processing in early visual analysis.

The visual system extracts rapidly the most important elements of the external world from a large flux of information, using early and intensive data reduction. If and how color information is used at this stage is still unknown. A recent model of early visual processing predicts - the features that human observers exploit for discrimination of compressed representations (sketches). [Del Viva et al, 2010. Journal of Vision, 10(7), 1360]; - that color does not provide a significant improvement in the information content of such sketches, compared to luminance alone [Punzi et al, 2010, Journal of Vision, 10(7), 432]. Here we measured human visual discrimination of briefly presented sketches (20 ms) containing either color or luminance information or both. Results show that performance obtained with equiluminant sketches (1-bit of color) is significantly lower (chance level) than that with dark/light grey sketches (1-bit of luminance). Adding an extra bit of color to the luminance bit does not increase performance, which is much lower than that obtained with 2-bits of luminance information. This suggests that early visual representations may not use color. Instead, color may be more suitable for a separate level of processing, following a rapid, initial luminance-based analysis.