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Scott H. Gabree, Rhea T. Eskew; Pedestal masking of S-cone increments and decrements: Less contrast gain control in the S-OFF pathways. Journal of Vision 2006;6(13):7. doi: 10.1167/6.13.7.
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Substantial evidence has accumulated to indicate that the mechanisms detecting S-cone increments (S+) and decrements (S−) differ in more than simply the polarities of their S, M and L cone inputs. This evidence includes S+ and S− differences with respect to: (1) changes in Ricco's area with eccentricity (Vassilev, et al., 2003); (2) different threshold elevations with opposite sawtooth flicker polarities (Shinomori, et al., 1999); (3) different field sensitivities in transient tritanopia (McLellan & Eskew, 2000); (4) different patterns of chromatic noise masking (Giulianini & Eskew, 2007; Wang & Eskew, 2007), and (5) different patterns of masking by pedestals (Vingrys & Mahon, 1998). We used unipolar, annular test and pedestal stimuli, presented in sawtooth pulses, designed to favor detection by S-ON or S-OFF mechanism. Like Vingrys and Mahon (1998), we measured test threshold versus pedestal contrast (TvC) functions for S+ and S− tests, but: (a) we used all four combinations of test and pedestal polarities, and (b) we also used long wave pedestals, consisting of equal increments (or decrements) of L and M cones (LM+ or LM−). When S-cone pedestals were used, classic “dipper” and “bumper” functions were found for near-threshold pedestals. Both S and LM pedestals produced masking at high pedestal contrasts; however, “bluish” (S+ or LM−) pedestals masked more than “yellowish” (S− or LM+) ones. The results indicate that these detection mechanisms are asymmetric, and suggest that perhaps there is a greater contrast gain control in the S-ON than the S-OFF pathway (Solomon & Lennie, 2005).
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