First-order color vision is slow; Second-order color vision is fast

Shapiro and colleagues (journalofvision.org/4/6/5 and 3/9/313) have developed a class of visual stimuli capable of separating the visual response to 1st-order information (i.e., chromaticity or luminance) from the response to 2nd-order information (i.e., chromatic or luminance contrast). The basic version of the stimulus consists of two physically identical disks, one surrounded by a dark annulus and the other by a light annulus. When the luminance levels of the disks are modulated sinusoidally in time, the 1st-order information from the disks modulates in phase, while the 2nd-order information modulates in antiphase. At 1 Hz, observers can track both the 1st- and 2nd-order information, creating the paradoxical impression that the disks modulate in antiphase but become light and dark at the same time. Analogous stimuli can be created for color. Here, we show that for both chromatic and luminance lights, the 2nd-order response predominates at higher temporal frequencies. This effect is not likely to be due to luminance artifacts since the intentional addition of luminance into the chromatic modulation eliminates the antiphase appearance. Further evidence for a fast chromatic contrast response comes from a novel experiment in which observers match the perceived modulation rate of a chromatically flickering light on a chromatic background (no contrast reversal) to the same light on a mid-white background (contrast reversal). Lights on the white background appear to modulate at twice the frequency of lights on the chromatic background, indicating the presence of a rectified chromatic contrast response. We propose the presence of two chromatic responses (1st and 2nd order). Chromatic thresholds are determined mostly by the 1st-order response; therefore, the 2nd-order chromatic response can be observed only at super-threshold levels. Luminance thresholds appear to be determined by a 2nd-order response. We will discuss the implications for color coding in the visual cortex.