The role of motion and number of element locations in mirror symmetry perception

Abstract

The human visual system has specialised mechanisms for encoding mirror-symmetry and for detecting symmetric motion-directions for objects that loom or recede from the observers. The contribution of motion to mirror-symmetry perception has never been investigated. Here we examine symmetry detection thresholds for stationary (static and dynamic flicker) and symmetrically moving patterns (inwards, outwards, random directions) with and without positional symmetry. We also measured motion detection and direction-discrimination thresholds for horizontal (left, right) and symmetrically moving patterns with and without positional symmetry. We found that symmetry detection thresholds were (a) significantly higher for static patterns, but there was no difference between the dynamic flicker and symmetrical motion conditions, and (b) higher than motion detection and direction-discrimination thresholds for horizontal or symmetrical motion, with or without positional symmetry. In addition, symmetrical motion was as easy to detect or discriminate as horizontal motion. We conclude that whilst symmetrical motion per se does not contribute to symmetry perception, limiting the lifetime of pattern elements does improve performance by increasing the number of element-locations as elements move from one location to the next. This may be explained by a temporal integration process in which weak, noisy symmetry signals are combined to produce a stronger signal.

Spatiotemporal and Luminance Contrast Properties of Symmetry Perception

“…Using dynamic dot-patterns refreshed at 65 Hz, Tyler et al [31,32] have investigated the effect of eccentricity on symmetry perception and found that static stimuli produced superior performance compared to dynamic stimuli, with static symmetry being perceived more quickly and further into the periphery. This is in contrast to more recent studies showing better performance with dynamic than static patterns [18,33]. Studies have shown that using dynamic stimuli consisting of rapid successive presentations of different symmetrical patterns improves symmetry detection [18] and orientation discrimination of the symmetry axis [33].…”

“…This is in contrast to more recent studies showing better performance with dynamic than static patterns [18,33]. Studies have shown that using dynamic stimuli consisting of rapid successive presentations of different symmetrical patterns improves symmetry detection [18] and orientation discrimination of the symmetry axis [33]. It has been suggested that these improvements in performance are due to multiple weak or noisy neural responses to symmetry being integrated over time to generate a stronger response.…”

“…Brain imaging (fMRI) studies have shown that symmetry involves a wide-spread network of extra-striate visual areas, such as V3a, V4, V7 and the lateral occipital complex (LOC) [11]. Although several recent studies have examined the contribution of visual features such as luminance polarity and colour [12][13][14], stereoscopic depth [15][16][17], and motion-direction [18] to symmetry perception, little is known about how spatiotemporal characteristics of dynamic patterns affect symmetry perception. Recent studies have shown that symmetry mechanisms can integrate spatial correlations across the symmetry midline within a~120 ms time window [19] and that limiting the lifetime of pattern elements improves symmetry detection thresholds [18].…”

“…All together, these studies suggest that the temporal integration window for symmetry might vary with the element lifetime duration and/or temporal frequency of dynamic stimulus presentation. Some indirect evidence for this comes from Sharman and Gheorghiu [18] study showing that symmetry detection thresholds were lower when elements lifetime durations were shorter. Therefore, here we will examine how symmetry detection thresholds are affected by element lifetime duration.…”

The neural basis of visual symmetry and its role in mid‐ and high‐level visual processing

“…Although detection of reflectional symmetry is not affected by number of elements, Sharman and Gheorghiu found that dynamic flicker or motion of the elements improves symmetry detection. This was consistent with previous work by Niimi et al .…”