Does perspective-distortion modulate the temporal tuning of symmetry responses?

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 viewpoint-invariant representation of objects, detection of regularity and structure, and mate selection. However, although symmetries are common in natural and artificial objects and scenes, they are subject to perspective-distortion and thus rarely give rise to symmetrical patterns on the retina during natural vision. 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 the effect of perspective-distortion on the temporal tuning of symmetry responses. We used novel, naturalistic 3D objects that had reflection symmetry over a vertical axis. The objects were procedurally generated along with well-matched control objects without any symmetries and then rendered to produce images in which object symmetries are either present in the image-plane or perspective-distorted. We measured visual system responses to image-plane and perspective-distorted symmetry 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 makes it possible to isolate symmetry-specific brain activity in the odd harmonics of the stimulation frequency. To investigate the temporal tuning of these responses, we used seven different stimulation frequencies in different conditions, between 1 and 10 Hz. We collected data from 30 participants with normal or corrected-to-normal visual acuity. We found that for both image-plane and perspective-distorted symmetry, responses peak at 2 Hz and are much reduced at higher frequencies across electrodes over occipital and temporal cortex. Response amplitudes were generally higher for image-plane symmetry, but surprisingly, the spatial tuning was not strongly modulated by perspective-distortion. Further investigations will determine how distinct visual regions may contribute to these results.