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.

Previous research has investigated the neural response to visual symmetry. It is well established that symmetry activates a network of extrastriate visual regions, including V4 and the Lateral Occipital Complex. This symmetry response generates an event-related potential called the sustained posterior negativity (SPN). However, previous work has used abstract stimuli, typically dot patterns or shapes. We tested the generality of the SPN. We confirmed that the SPN wave was present and of similar amplitude for symmetrical shapes, flowers and landscapes, whether participants were responding either to image symmetry or to image color. We conclude that the extrastriate symmetry response can be generated by any two-dimensional image and is similar in different stimulus domains.

Human perception of symmetry is associated with activation in an extended network of extrastriate visual areas. This activation generates an ERP called the Sustained Posterior Negativity (SPN). In most studies so far, the stimuli have been defined by luminance. We tested whether the SPN is present when stimuli are defined by stereoscopic disparity using random dot stereograms (RDS). In Experiment 1, we compared the SPN signal for contours specified by binocular disparity and contours specified by monocular cues. The SPN was equivalent, suggesting that the type of contour does not alter the SPN signal. In Experiment 2 we exploited the unique property of RDS to provide unambiguous figure-ground arrangements. Psychophysical work has shown that symmetry is more easily detected when it is a property of a single object (i.e., within a figure), compared to a property of a gap between two objects (i.e., the ground). Therefore, the target regions in this experiment could either be foreground or background. The SPN onset was delayed when the symmetry was in a ground region. This may be because object formation interferes with the processing of shape information in the ground region.

The brain must identify objects from different viewpoints that change the retinal image. This study examined the conditions under which the brain spends computational resources to construct view-invariant, extraretinal representations in a 3D virtual environment. We focused on extraretinal representation of visual symmetry. Visual symmetry activates the extrastriate visual cortex and generates an Event Related Potential (ERP) called Sustained Posterior Negativity (SPN). Amplitude is lower for symmetrical compared to asymmetrical stimuli at posterior electrodes. Given a symmetric pattern on a plane, regularity in the retinal image is degraded by perspective. Previous studies have found that the SPN is selectively reduced for perspective symmetry - we term this perspective cost. This cost may be reduced when sufficient visual cues are available to support view invariance. To explore this, we used a VR-based 3D environment. Forty-eight participants completed 2 tasks, discriminating stimulus regularity (symmetry or asymmetry) and discriminating stimulus luminance (light or dark). We computed perspective cost as the difference between frontoparallel and perspective SPN. In the Regularity task, perspective cost was significantly <.35 μV - our a priori definition of a small SPN modulation, indicating no perspective cost. The results from the Luminance task were less clear. SPN cost was not significantly more than 0 μV, but not significantly less than .35 μV. We conclude that the extrastriate cortex can construct extraretinal representations of symmetry when sufficient visual depth cues are available. This certainly happens during regularity discrimination and may happen automatically during luminance discrimination.

Objects project different images when viewed from varying locations, but the visual system can correct perspective distortions and identify objects across viewpoints. This study investigated the conditions under which the visual system allocates computational resources to construct view-invariant, extraretinal representations, focusing on planar symmetry. When a symmetrical pattern lies on a plane, its symmetry in the retinal image is degraded by perspective. Visual symmetry activates the extrastriate visual cortex and generates an Event Related Potential (ERP) called Sustained Posterior Negativity (SPN). Previous research has shown that the SPN is reduced for perspective symmetry during secondary tasks. We hypothesized that perspective cost would decrease when visual cues support extraretinal representation. To test this, 120 participants viewed symmetrical and asymmetrical stimuli presented in a frontoparallel or perspective view. The task did not explicitly involve symmetry; participants discriminated the luminance of the patterns. Participants completed four experimental blocks: (1) Baseline block: no depth cues; (2) Monocular viewing block: stimuli viewed with one eye; (3) Static frame block: pictorial depth cues from elements within a flat surface with edges; (4) Moving frame block: motion parallax enhanced 3D interpretation before stimulus onset. Perspective cost was calculated as the difference between SPN responses to frontoparallel and perspective views. Contrary to our pre-registered hypotheses, the perspective cost was consistent across all four blocks. We conclude that the tested visual cues do not substantially reduce the computational cost of processing perspective symmetry.

The extrastriate visual cortex is activated by visual regularity and generates an ERP known as the sustained posterior negativity (SPN). Spatial filter models offer a biologically plausible account of regularity detection based on the spectral properties of an image. These models are specific to reflection and therefore imply that reflectional symmetry and Glass patterns are coded by different neural populations. We utilised the SPN priming effect to probe representational overlap between reflection and Glass patterns. For each trial, participants were presented with a rapid succession of three patterns. In the Repeated condition, three reflections or three Glass patterns were presented. In the Changing condition, patterns alternated between reflection and Glass patterns. An increase in SPN amplitude (priming) was observed in both the Repeated and Changing conditions. Results indicate a greater representational overlap in the brain between reflection and Glass patterns than predicted by spatial filter models.