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Branka Spehar, Luke Vu; Contour Shape Processing: Contrast Polarity and Perceived Aspect Ratio. Journal of Vision 2010;10(7):1199. doi: 10.1167/10.7.1199.
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© ARVO (1962-2015); The Authors (2016-present)
The visual system parses the visual scene into objects and their spatial locations by integrating local signals into global object representations. We have shown that spatial distribution of luminance polarities in an image plays an important role in the processes of perceptual unit formation (Spehar, 2000; Spehar & Clifford, 2003). For example, a pronounced degradation of perceived illusory shapes is observed in configurations with contrast polarity variations at the intersections of orthogonally oriented edges within inducing configurations, but not in configurations where contrast polarity variations did not coincide with points of highest orientation discontinuity. A similar effect of the distribution of contrast polarity reversals was demonstrated in perceptual closure: the presence of contrast polarity reversals impaired visual search for shapes with contrast polarity reversal positioned at points of high contour curvature of the bounding contour (Spehar, 2002). Here we report an analogous effect of the distribution of contrast polarity reversals on the computation of aspect ratio of centrally presented contour shapes. The perceived aspect ratios appear more vertically elongated when contrast polarity changes at points of intersection of differently oriented component lines (corners), compared to configurations with no contrast polarity variations and configurations where contrast polarity variations appear at the collinear line segments. These results suggest an important and different role of contrast polarity in combining smooth parts of object contours with those at points of high curvature. We believe that this pervasive sensitivity to the distribution of contrast polarity reversals along bounding contours is related to the processes that mediate grouping at intersections of lines of different orientation. If contrast polarity reverses at such junctions the probability that these segments will be grouped together is decreased which in turn influences the mechanisms involved in computation of shape area.
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