The spatial mechanisms mediating symmetry perception

This paper examines the role of spatial frequency and orientation tuned channels in the perception of visual symmetry. Subjects discriminated between band-pass filtered, white noise textures that either did or did not contain vertical bilateral symmetry (VBS, i.e., around a vertical midline) as a function of the spatial phase disruption imposed on the images. Resistance to phase noise is largely scale-invariant for isotropically filtered images, but horizontally filtered images are consistently more noise-resistant than vertical. However, when stimuli are rotated through 90 deg (horizontal bilateral symmetry, HBS) performance is better with vertically filtered images suggesting a general advantage for orientations orthogonal to the axis of symmetry. At these orientations symmetry may be signaled directly by clusters of features along the axis. Our data further suggest that the established disadvantage for HBS may be attributable to an over-reliance on the output of horizontal filters. We compare models which exploit feature clustering around the axis by measuring the co-alignment in the output of oriented filters. Models using filters oriented orthogonal to the axis of symmetry predict the psychophysical performance for isotropic patterns and for patterns filtered orthogonal to the axis. For patterns filtered parallel to the axis, our data suggest that visual attention may play a role.