Cone contrast comparison of luminance and chromatic sensitivities for movement, flicker, and flashes

Abstract

Expressing visual stimuli in terms of the contrast they produce in the different cone classes approximately accounts for (1) the highly overlapping cone spectra and (2) (von Kries) adaptation, allowing meaningful comparisons of post-receptoral sensitivities to color and luminance. This paper reviews this comparison for lights that stimulate only the L and N cones (ca. 3 log Td adapt). For low to medium temporal and spatial frequencies, chromatic sensitivity is as much as 9× luminance sensitivity. For example, for 1° flickering spots chromatic sensitivity is higher than luminance sensitivity below 15 Hz. Experiments using moving 1 cpd gratings reveal two spectrally-opponent (L–M) mechanisms and a luminance mechanism. The color detection mechanism is 5 × more sensitive than the luminance mechanism at 1°/s, while the spectrally-opponent motion mechanism (which apparently does not signal color) is 3× more sensitive than the luminance mechanism. Greater temporal integration in spectrally opponent mechanisms may partly account for their greater sensitivity. With 1° flashes, the luminance and chromatic critical durations are about 40 and 100 ms, respectively. However, even for the briefest flashes, chromatic sensitivity is 3× luminance sensitivity. Part of this 3× could be due to the differencing operation reducing noise in opponent mechanisms; however, this hypothesis requires considerable correlated noise in the L and M cones.