No abstract

Symmetry is a complex image property that is exploited by a sufficiently wide range of species to indicate that it is detected using simple visual mechanisms. These mechanisms rely on measurements made close to the axis of symmetry. We investigated the size and shape of this integration region (IR) by measuring human detection of spatially band-pass symmetrical patches embedded in noise. Resistance to disruption of symmetry (in the form of random phase noise) improves with increasing patch size, and then asymptotes when the embedded region fills the IR. The size of the IR is shown to vary in inverse proportion to spatial frequency; i.e. symmetry detection exhibits scale invariance. The IR is shown to have rigid dimensions, elongated in the direction of the axis of symmetry, with an aspect ratio of ca. 2:1. These results are consistent with a central role for spatial filtering in symmetry detection.

No abstract

Bilateral or mirror symmetry is a ubiquitous feature of biological forms that the visual system could exploit for segmenting an object from a cluttered background. If this is so, the visual system may be prepared to detect symmetry at all retinal locations in parallel. Indeed, a biologically plausible model that responds optimally at axes of symmetry is quite easy to construct. Our data show, however, that if such a mechanism exists, it works with high efficiency only at the fovea. The detection of vertical bilateral symmetry embedded in random noise is very poor unless the axis of symmetry is very close to the point of fixation. This leads to the conclusion that symmetry does not play an important role in image segmentation and that it is important to the visual system only after it is fixated.