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Elias Cohen, Qasim Zaidi; Salience of mirror symmetry in natural patterns. Journal of Vision 2007;7(9):970. doi: https://doi.org/10.1167/7.9.970.
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© ARVO (1962-2015); The Authors (2016-present)
Many studies suggest that axes of mirror symmetry are salient visual structures perceived effortlessly. While this may be true for classes of noise or dot stimuli used in previous studies of symmetry perception, it is unknown whether this is true when the underlying pattern also contains structure. This study quantifies the salience of symmetry across a variety of patterns and identifies the determining factors. Observers were presented with 101 classes of symmetric pattern, generated by reflecting and adding Brodatz textures. The minimum presentation duration required to reliably identify the axis of symmetry as vertical or horizontal was measured by randomly interleaved 2AFC staircases. Thresholds for texture types varied between 40 and 400 ms, showing that salience of symmetry depends on the underlying pattern. Using image analysis, we identified two factors that underlie this dependence. First, if the structure at the symmetry axis is similar to the structure in the rest of the image, longer durations are required to identify the axis. Second, a comparison of the pooled responses of V-shaped filters oriented at, versus orthogonal to and surrounding, the axis of symmetry correlated with salience of the axis. Taken together these two factors explain more than 50% of the variance in duration thresholds. Since V-shaped filters pick up mirror orientations across the axis, their success demonstrates that mirror orientations are the major component of pattern symmetry. Combinations of V-shaped filters of different angles would also be suited to extracting 3-D shape from texture orientation flows, suggesting a common neural substrate for two different visual functions.
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