Nikan Movahedi^1,2 (), Peter J. Kohler^1,2; ¹Department of Psychology, York University, Toronto, ON, ²Center for Vision Research, York University, Toronto, ON

Symmetry is a highly salient feature in both natural and man-made environments. Numerous species are sensitive to symmetry, and symmetry is thought to be an important cue for visual tasks, including figure-ground segmentation, shape detection, and viewpoint-invariant representation of objects. However, while symmetries are common in natural and artificial objects and scenes, in natural vision, they are often distorted by perspective and rarely produce symmetrical patterns on the retina. Here, we build on previous studies showing that perspective-distortion makes symmetry responses weaker and more task-dependent (Makin et al., 2014; Keefe et al., 2018) by investigating its effect on the temporal tuning of symmetry responses. We used novel, naturalistic 3D objects with reflection symmetry about a vertical axis. Symmetric objects and well-matched asymmetric controls were procedurally generated and rendered to produce images in which object symmetries are either present in the image-plane or subject to perspective-distortion. Across 3 studies (N=95), we measured visual system responses to both image-plane and perspective-distorted symmetries using high-density EEG with a Steady-State Visual Evoked Potentials (SSVEPs) paradigm, in which images of symmetrical objects alternate with images of control objects. This allows isolation of symmetry-specific brain activity in the odd harmonics of the stimulation frequency. We investigated the temporal tuning of these responses across four experiments using seven stimulation frequencies between 1 and 10Hz. We found that responses to image-plane and perspective-distorted symmetries generally shared the same temporal tuning, with slower frequencies (1-2 Hz) producing the highest amplitude responses. Perspective-distorted symmetries produces responses that were weaker overall, consistent with prior work, and more right-lateralized, compared to image-plane symmetry. Our results suggest that image-plane and perspective-distorted symmetry is detected by mechanisms that share the same temporal tuning, but may differ in the contribution of the two hemispheres.

Acknowledgements: This work was supported by the Vision Science to Applications (VISTA) program funded by the Canada First Research Excellence Fund (CFREF, 2016–2023) and by a Discovery Grant from the Natural Sciences and Engineering Research Council of Canada awarded to PJK.