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Stuart Anstis, Patrick Cavanagh; A line-doubling illusion. Journal of Vision 2021;21(9):2059. doi: https://doi.org/10.1167/jov.21.9.2059.
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© ARVO (1962-2015); The Authors (2016-present)
We present a novel ‘dazzle’ illusion in which a bar has the reverse polarity of the oblique square-wave grating on which it lies. When this bar is viewed in near peripheral vision, it can look doubled, as if there were two bars side by side and touching. During direct fixation the bar is seen veridically. But in near peripheral vision, say when one looks at the edge of the background, the bar looks doubled, like two bars side by a side, touching each other making three vertical edges (the middle one shared by the two bars). We examined two possible factors that could contribute to the effect – blurring and phase insensitivity of the periphery – but found that they played no role. We also asked whether it was contrast reversal or phase shift cause the line doubling. In the square wave grating, this contrast reversal that defines the bar is also a 180° phase shift relative to the background grating. To separate the two, we used pin-striped and ramp gratings where a 180° phase shift is quite different from a contrast reversal. In both cases, when the bar is phase shifted it appears doubled, but when it is contrast reversed it does not. We conclude that spatial phase reversal, not contrast reversal, is responsible for perceptual doubling.
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