Brain Responses to Symmetries in Naturalistic Novel Three-Dimensional Objects

Symmetries are prevalent in natural and man-made objects and scenes. During natural vision, symmetries in the world are subject to perspective-distortion and rarely produce symmetrical images on the retina. Here we used high-density EEG to investigate the ability of the human visual system to overcome such distortions, by measuring responses to images of naturalistic, novel, 3D objects. Our paradigm used Steady-State Visual Evoked Potentials (SSVEPs) to isolate brain activity specific to symmetry processing. We presented images of symmetrical and asymmetrical objects under two viewing conditions: One that produced symmetries in the image plane, and another where objects were rotated such that symmetries would be distorted in the resulting image. In each stimulus cycle, an asymmetrical object image was shown followed by a second image, either symmetrical or, in a control condition, another asymmetrical image. Image pairs for each cycle were selected so activity in a deep convolutional neural network trained to classify object categories (VGG16; Simonyan and Zisserman, 2015) was similarly matched between all image-pairs. We showed 10 cycles per trial, at a stimulation frequency of 1 Hz. In a follow-up experiment, the shading cues to 3D shape were removed from the images. In the resulting 2D object silhouettes, perspective-distorted symmetry cannot be detected. We analyzed the SSVEP data in three electrode regions-of-interest over occipital cortex and left and right temporal cortex, defined based on independent data. The results showed that during passive viewing, perspective-distorted symmetry can elicit measurable symmetry-specific SSVEPs, but compared to image-plane symmetry they are weaker, more anterior and possibly more right-lateralized, consistent with responses in higher-level visual cortex. The 2D stimuli elicited similar responses to 3D for image-plane symmetry, but as expected, produced no symmetry responses for perspective-distorted symmetry. Future work will determine how task manipulations may influence responses to these stimuli.