Spectral response of contrast-flash masking

Abstract

Parafoveal spectral response functions for the contrast-flash masking of a just-visible 661-nm bar test are proportional to Π5 (Stiles’s long-wave function) at long wavelengths but show an unexpected dip around 575 nm and are almost flat at short wavelengths, on a dim (2-Td) rod-desensitizing 500-nm auxiliary. This effect is not due to rod intrusions or scattered light. Masking functions are approximately proportional to Π5 throughout the spectrum, however, if the 500-nm auxiliary is replaced by a π5-equated 445-nm auxiliary or if the flanking bar mask used in the main experiment is replaced by a uniform contrast-flash mask. These results suggest that a contour-sensitive interaction involving short-wave cones may modify the extent of masking in the pathway that detects the long-wave test.