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Boris Sheliga, Christian Quaia, Edmond FitzGibbon, Bruce Cumming; Human contrast normalization process operates on a local scale. Journal of Vision 2015;15(12):287. doi: 10.1167/15.12.287.
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© 2017 Association for Research in Vision and Ophthalmology.
We quantitatively assessed the strength and extent of spatial summation in human Ocular Following Responses (OFRs). In Experiment 1 we used a pair of 1D sine wave gratings which both drifted horizontally in the same direction but whose vertical or horizontal spatial separation was manipulated. We compared the OFRs to the pair with responses recorded when each grating, comprising the pair, was presented in isolation. Overall, as the separation between the gratings increased, the interaction progressed from sub-linear summation towards pure summation and even facilitation (in 2 out of 3 subjects), confirming earlier observations (Quaia et al, 2012). Additionally, the strength of the suppressive interactions at each separation tested was considerably weaker in the horizontal than in the vertical stimulus configuration. In Experiment 2 we kept the pair’s spatial separation constant while independently varying the contrast of each grating. For a fixed contrast of one grating ("conditioning grating") we described responses as a function of contrast in the other grating ("test") with a Naka-Rushton equation. Changing only Rmax in these fits as a function of contrast in the conditioning grating produced excellent fits. The changes in fitted response amplitude accounted for 92±4% of changes in observed OFR amplitude. Conversely, when only C50 was changed with conditioning contrast, the fits were poor and accounted for just 14±9% of changes in observed OFR amplitude. If the conditioning grating affected contrast normalization for the test changes in C50 should be expected. Thus, this pattern of results suggests that contrast normalization operates on a local scale.
Meeting abstract presented at VSS 2015
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