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Scott H. Gabree, Jr., Rhea T. Eskew; Asymmetric pedestal masking of S-cone increments and decrements: Does sawtooth polarity matter?. Journal of Vision 2007;7(9):672. doi: 10.1167/7.9.672.
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There is substantial evidence to indicate that the mechanisms detecting S-cone increments (S+) and decrements (S−) differ in more than just the polarities of their S, M and L cone inputs. Our lab has investigated the S-cone increment and decrement pathways using a pedestal method. We used S+ and S− pedestals as well as an equal combination of L and M-cone increments or decrements that had a color appearance similar to S-cone stimuli. 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. Consistent with other results (Vingrys & Mahon, 1998), these results indicate that these detection mechanisms are asymmetric (McLellan & Eskew, 2000; Shinomori, Spillman & Werner, 1999), and suggest that perhaps there is a greater contrast gain control in the S-ON than the S-OFF pathway (Solomon & Lennie, 2005).
All stimuli (pedestals and tests) in these experiments used a “rapid-start” temporal waveform, one that rapidly increased (or decreased) in contrast and then linearly ramped back to the mean field. The incremental or decremental stimulus contrasts, coupled with “rapid-start” temporal waveforms, should differentially stimulate S-ON and S-OFF pathways. The question we ask here is whether the temporal sawtooth polarity is important in creating the asymmetry we found. We repeated the pedestal masking experiment, but with a “rapid-stop” temporal waveform that linearly ramped to a peak and then abruptly returned to the mean field. Tests were again S+ or S−; pedestals were S+, S−, LM+, or LM− as before. Even with high contrast pedestals, the sawtooth polarity had little effect. One possible interpretation is that the contrast polarity difference produces clear separation of ON and OFF pathways, and changing the sawtooth polarity of single flashes has little further effect.
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