Asymmetric high-contrast masking in S cone increment and decrement pathways

Physiological, anatomical, and psychophysical evidence points to important differences between visual processing of short-wave cone increments and decrement (S+ and S−) stimuli. The present study uses the pedestal discrimination paradigm to investigate potential differences, using S+ and S− tests presented on (L)ong-wave, (M)edium-wave, S, L+M, L−M, and achromatic pedestals, of both contrast polarities. Results show that high contrast ‘purplish’ (S+ or −(L+M)) pedestals produce substantially more masking of both S+ and S− tests than ‘yellowish’ (S− or +(L+M)) pedestals do. The other pedestals produce no masking. These findings suggest greater nonlinearity – either a static nonlinearity or contrast gain control – in the mechanisms responsible for the ‘purplish’ polarity, likely the S ON pathway.