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

We investigated perceptual interactions between orientation selective corti cal filters using a task in which the observer was to detect mirror symmetr y in briefly flashed visual patterns composed of oriented Gabor elements. O n each trial, the Gabor patches were randomly placed in one half of the sti mulus region, and symmetry was generated by reflecting the positions of the patch centers across the vertical axis. A prespecified proportion of patch es were in mirror symmetrical positions with the remaining patches placed a t random positions. The perception of mirror symmetry was measured for thre e stimulus conditions: (1) same orientation (i.e. all the local Gabor eleme nts were either vertical or horizontal), (2) mixed matching (i.e. the orien tations 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, bu t in a mirror symmetrical pair they were always orthogonal). We found that the perception of global symmetry was poorer (thresholds were elevated) whe n the local orientations of feature pairs were orthogonal than when they we re matched. This result is consistent with the properties of the neurons in the corpus callosum, which selectively interconnect cortical filters with identical orientation specificity. NeuroReport 11:2133-2138 (C) 2000 Lippin cott Williams & Wilkins.