The extrastriate symmetry response is robust to alcohol intoxication.

Visual symmetry activates a network of regions in the extrastriate cortex and generates an event-related potential (ERP) called the sustained posterior negativity (SPN). Previous work has found that the SPN is robust to experimental manipulations of task, spatial attention, and memory load. In the current study, we investigated whether the SPN is also robust to alcohol-induced changes in mental state. A pilot experiment (N = 13) found that alcohol unexpectedly increased SPN amplitude. We followed this unexpected result with two new experiments on separate groups, using an alcohol challenge paradigm. One group completed an Oddball discrimination task (N = 26). Another group completed a Regularity discrimination task (N = 26). In both groups, participants consumed a medium dose of alcohol (0.65 g/kg body weight) and a placebo drink, in separate sessions. Alcohol reduced SPN amplitude in the Oddball task (contrary to the pilot results) but had no effect on SPN amplitude in the Regularity task. In contrast, the N1 wave was consistently dampened by alcohol in all experiments. Exploratory analysis indicated that the inconsistent effect of alcohol on SPN amplitude may be partly explained by individual differences in alcohol use. Alcohol reduced the SPN in light drinkers and increased it in heavier drinkers. Despite remaining questions, the results highlight the automaticity of symmetry processing. Symmetry still produces a large SPN response, even when participants are intoxicated, and even when symmetry is not task relevant.

Neural responses to reflection symmetry for shapes defined by binocular disparity, and for shapes perceived as regions of background.

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 extrastriate symmetry response can be elicited by flowers and landscapes as well as abstract shapes.

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.