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Enrico Giora, Simone Gori, Arash Yazdanbakhsh, Ennio Mingolla; The Accordion Grating illusion measured by a nulling paradigm. Journal of Vision 2010;10(7):821. doi: 10.1167/10.7.821.
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© ARVO (1962-2015); The Authors (2016-present)
Dynamical viewing of an elementary square-wave grating by moving toward it creates the Accordion Grating illusion. Observers report a nonrigid perceptual distortion of the grating including two illusory effects: (i) an expansion only perpendicular to the stripes when moving the head towards the pattern and (ii) distortion of the physically straight stripes of the grating. Whilst the illusory expansion perpendicular to the stripes can be explained by the interactions between ambiguous and unambiguous motion signals generated at line interiors and line ends, a differential geometry model with a 3-D representation of the classical aperture problem is here proposed to account for the illusory curvature. Four subjects were tested in a nulling psychophysical experiment. The expected perceptual curvature was balanced by a physical counter-distortion calculated by the model. The amount of physical curvature necessary to nullify the illusion led to a precise quantification of the illusion and verified the proposed model.
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