The role of vertical mirror symmetry in visualshape detection

Abstract

The goal of our study is a better understanding of the role of vertical mirror symmetry in perceptual grouping. With a simple psychophysical task and a set of controlled stimuli, we investigated whether vertical mirror symmetry acts as a cue in figure-ground segregation. We asked participants to indicate which of two sequentially presented Gabor arrays contained a visual shape. The shape was defined by a subset of Gabor elements positioned along the outline of an unfamiliar shape. By adding orientation noise to these Gabor elements, the shape percept became less salient. Across the different noise levels, symmetric shapes were easier to detect than asymmetric ones. This finding indicates that vertical mirror symmetry is indeed used as a cue in perceptual grouping.

Traditional and New Principles of Perceptual Grouping

“…Such symmetrical patterns have been found to be particularly visually salient. For instance, symmetry has clear effects on detection of patterns in random dot fields, contours, and other stimuli (e.g., Machilsen, Pauwels, & Wagemans, 2009;Norcia, Candy, Pettet, Vildavski, & Tyler, 2002;. However, when a symmetrical pattern is tilted relative to the frontal plane, its features in the image projected to the retinae are no longer symmetrical.…”

Symmetry Detection in Visual Impairment: Behavioral Evidence and Neural Correlates

“…Moreover, the level of activity in extrastriate regions was found to positively correlate with the likelihood of symmetry detection [27]. The recruitment of LOC in visual symmetry detection may at least partially depend on the role of this region in object recognition [58,59] being that symmetry is an important cue in shape/object detection [5]. However, control experiments carried out by Sasaki et al [27] suggest that areas V3A, V4d/v, V7 and LO responded predominately to symmetry and not to general object-like features presented in the stimuli.…”

“…Psychophysical experiments mainly employing simple shapes or dense dot patterns as stimuli have shown that symmetry can be detected extremely quickly (within a few tens of milliseconds) and in an automatic (i.e., not mediated by a conscious cognitive effort; see [1] for review), pre-attentive manner [2][3][4]. Symmetry acts as a grouping principle of perceptual organization [5]: elements sharing symmetry relations tend to be aggregated, thus facilitating figure-ground segregation. It has also been suggested that symmetry detection may be a special case of orientation processing [6,7] in that once the axis of symmetry is determined, both this information and orientation information acquired from other contour classes converge at a common neural site where orientation information is processed irrespective of source.…”

Peripheral Contour Grouping and Saccade Targeting: The Role of Mirror Symmetry

“…There is indeed evidence for stronger or faster integration for closed versus open contours [22,24] (see [25] for an important caveat) and recently Machilsen et al [18] showed a beneficial effect of symmetry on contour integration in central vision. In their experiments, the detection of closed contour shapes in briefly presented stimuli-presentation duration varied from 100 to 350 ms, adjusted for each observer via an adaptive procedure during the training phase of the experiment-was more robust to the jittering of the orientations of the individual contour elements for symmetric than for asymmetric contours.…”

“…Machilsen, Pauwels and Wagemans [18] were the first to investigate the role of vertical mirror symmetry in perceptual grouping using the modern psychophysical paradigm of contour integration. Contour integration refers to the process by which paths of Gabor elements embedded in distracter Gabors are perceptually grouped provided they are in sufficiently close proximity to each other and have orientations and positions consistent with a smooth contour (e.g., [19][20][21][22]; see [23] for a recent review).…”