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John Cass; Mutual effects of orientation and contrast within and between the eyes: from summation to suppression. Journal of Vision 2010;10(7):1374. doi: https://doi.org/10.1167/10.7.1374.
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© ARVO (1962-2015); The Authors (2016-present)
Previous masking studies estimate the bandwidths of orientation-selective mechanisms based on the rate at which threshold elevations decrease as a function of the angular difference between spatio-temporally overlaid target and masking stimuli, with many finding that bandwidths decrease with increasing spatial frequency. In this study we examine, in detail, the effects of relative phase, contrast, and the ocular mode of presentation (MON vs DICH) on the relationship between orientation masking and spatial frequency. Results: When relative phase was randomized across trials we observed similar masking functions to those observed previously. When relative phase was blocked, however (ie. in phase or anti-phase), monoptic thresholds increased up to ∼10 degrees of angular difference, then decreased monotonically out to 90 degrees. This qualitative pattern, evident at all mask contrasts (0.5 − 64 x threshold), is well fitted by an orientation-defined Difference of Gaussian (DoG) model, consisting of narrowband summation (∼5-10 degrees) combined with a broader suppressive component which becomes narrower with increasing spatial frequency. Interestingly, identical non-monotonic patterns were observed dichoptically at low mask contrasts (0.5 - ∼8 x threshold). At higher mask contrasts however, the sign of the narrowband component reversed to produce what appeared to be narrowband inter-ocular suppression (Baker & Meese, 2007), whilst the broader suppressive component remained almost identical to that derived monoptically. These results indicate that human orientation channels laterally inhibit one another at a neural locus that receives binocular input, with the extent of lateral inhibition decreasing with spatial frequency. Whilst narrowband summation is evident within and between the eyes, narrowband suppression results when inter-ocular contrast differences are high, possibly to reduce inter-ocular redundancy.
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