Symmetry Perception and Psychedelic Experience

A review of symmetry perception synthesizes findings from psychophysics, neuroscience, and aesthetics. Over a century of psychophysical studies are joined by recent functional MRI and EEG evidence showing that regular visual patterns—reflection symmetry, Glass patterns, and wallpaper groups—activate the extrastriate visual cortex, producing an event-related potential called sustained posterior negativity (SPN). SPN amplitude scales with the degree of visual regularity and occurs regardless of attention. Some symmetry detection appears automatic, unconscious, and pre-attentive, possibly hardwired, which may explain its aesthetic appeal. Symmetry and fractal geometry also appear in hallucinations induced by DMT and visual flicker, suggesting the extrastriate cortex can generate symmetrical representations spontaneously without external stimuli.

Study at a glance

Abstract

This review of symmetry perception has six parts. Psychophysical studies have investigated symmetry perception for over 100 years (part 1). Neuroscientific studies on symmetry perception have accumulated in the last 20 years. Functional MRI and EEG experiments have conclusively shown that regular visual arrangements, such as reflectional symmetry, Glass patterns, and the 17 wallpaper groups all activate the extrastriate visual cortex. This activation generates an event-related potential (ERP) called sustained posterior negativity (SPN). SPN amplitude scales with the degree of regularity in the display, and the SPN is generated whether participants attend to symmetry or not (part 2). It is likely that some forms of symmetry are detected automatically, unconsciously, and pre-attentively (part 3). It might be that the brain is hardwired to detect reflectional symmetry (part 4), and this could contribute to its aesthetic appeal (part 5). Visual symmetry and fractal geometry are prominent in hallucinations induced by the psychedelic drug N,N-dimethyltryptamine (DMT), and visual flicker (part 6). Integrating what we know about symmetry processing with features of induced hallucinations is a new frontier in neuroscience. We propose that the extrastriate cortex can generate aesthetically fascinating symmetrical representations spontaneously, in the absence of external symmetrical stimuli.