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Maria Pereverzeva, Scott O. Murray; Luminance gradient configuration determines perceived lightness in a simple geometric illusion . Journal of Vision 2014;14(10):74. doi: https://doi.org/10.1167/14.10.74.
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© ARVO (1962-2015); The Authors (2016-present)
Accurate perception of surface reflectance poses a significant computational problem for the visual system. The amount of light reflected by a surface is affected by many factors including the surface's reflectance properties and illumination conditions. The latter are not limited by the strength of illuminant but also include the relative placement of light illuminating the surface, the orientation of the surface and its 3d shape, all of which result in a pattern of luminance gradients across the surface. In this study we explore how the presence of luminance gradients in parts of the image contributes to lightness perception. We introduce a novel, simple lightness illusion. It consists of six separate checks, organized in rows of two. Each check has a positive luminance gradient across it. The top and the bottom rows are the same: with the darker check on the left, and the lighter check on the right. Two checks in the middle row are identical; however, the right check appears darker than the left. 3-7 subjects participated in 10 conditions of the experiment, with various stimulus configurations. The illusory "darker" check was perceived on average 12% IL (IL=100%*(L-Lmin)/(Lmax-Lmin), L=stimulus luminance, Lmin=monitor black level, Lmax=display maximal luminance) darker than the veridical luminance match. Reversing the gradient orientations of the middle row checks eliminated the illusion. Surprisingly, the illusion was still present, (even though decreased, at about 5%IL,) when the top and the bottom row were substituted by identical, spatially uniform gray checks. As there are no shared borders between the checks, simultaneous contrast cannot explain the effect. However, there are multiple possible explanations including spatial filtering (Blakeslee & McCourt, 2004) or by some higher-order mechanism such as perceptual grouping or amodal completion. We explore these possibilities by manipulating the luminance configurations and the gradient slopes of the checks.
Meeting abstract presented at VSS 2014
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