Perception of mirror symmetry reveals long-range interactions between orientation-selective cortical filters.

We investigated perceptual interactions between orientation selective cortical filters using a task in which the observer was to detect mirror symmetry in briefly flashed visual patterns composed of oriented Gabor elements. On each trial, the Gabor patches were randomly placed in one half of the stimulus region, and symmetry was generated by reflecting the positions of the patch centers across the vertical axis. A prespecified proportion of patches were in mirror symmetrical positions with the remaining patches placed at random positions. The perception of mirror symmetry was measured for three stimulus conditions: (1) same orientation (i.e. all the local Gabor elements were either vertical or horizontal), (2) mixed matching (i.e. the orientations could be randomly both vertical and horizontal with the constraint that the orientations in a mirror symmetrical pair were matching), and (3) mixed opposing (i.e. the orientations were both vertical and horizontal, but in a mirror symmetrical pair they were always orthogonal). We found that the perception of global symmetry was poorer (thresholds were elevated) when the local orientations of feature pairs were orthogonal than when they were matched. This result is consistent with the properties of the neurons in the corpus callosum, which selectively interconnect cortical filters with identical orientation specificity.