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Martin Berg, James A. Schirillo, Michael Kubovy; Illuminant complexity and grouping by proximity. Journal of Vision 2002;2(7):470. doi: https://doi.org/10.1167/2.7.470.
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Rock et al. (Perception, 1992, 21, 779–789) showed that grouping can be computed post-constancy, after discounting a complex scene's variations in illumination. Unpublished studies by Kubovy and his co-workers have shown grouping among heterogeneous dots; hence, constant lightness is not a prerequisite for grouping. These experiments suggest that illuminant complexity can affect the discounting of variations in illumination. Our stimuli were rectangular dot-lattices presented at random orientations in a circular aperture. In these lattices |a| is the shortest inter-dot distance, which we held constant, and |b| is the second shortest inter-dot distance, which we varied. We convolved (a) lattices in which the dots and the background were of uniform luminance with (b) sinusoidal luminance gratings, while keeping the luminance ratio of background to dots constant at 1.2. The stripes of the gratings were parallel to the orientation of b. We used a high (3.37 c/d) and low (0.017 c/d) spatial frequency grating (while keeping the maximal change in luminance per pixel constant) to see if observers would disregard the sinusoidal “shadow.” The observers were given a phenomenological 4AFC to indicate the grouping in the dot lattice,in three conditions: (i) no grating and (ii) high- and (iii) low-frequency gratings. Discounting did not occur with a high spatial frequency grating (grouping was biased by the orientation of the stripes), but it did occur with a low spatial frequency grating (grouping was not different from grouping in the no-grating condition). This suggests that a complex, non-monotonic, shadow cannot be “discounted,” preventing constancy from occurring, whereas a monotonic shadow allows for post-constancy grouping. [Supported by NEI grant No. R01 EY 12926 to the University of Virginia.]
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