Perspective Slant Makes Symmetry Harder to Detect and Less Aesthetically Appealing

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.

When does perceptual organization happen?

Reflectional (mirror) symmetry is an important visual cue for perceptual organization. The brain processes symmetry rapidly and efficiently. Previous work suggests that symmetry activates the extrastriate cortex and generates an event related potential (ERP) called the Sustained Posterior Negativity (SPN). It has been claimed that no tasks completely block symmetry processing and abolish the SPN. We tested the limits of this claim with a series of eight new Electroencephalography (EEG) experiments (344 participants in total). All experiments used the same symmetrical or asymmetrical dot patterns. When participants attended to regularity in Experiment 1, there was a substantial SPN (Mean amplitude = -2.423 μV). The SPN was reduced, but not abolished, when participants discriminated dot luminance in Experiments 2 and 3 (-.835 and -1.410 μV) or the aspect ratio of a superimposed cross in Experiments 4 and 5 (-.722 and -.601 μV). The SPN also survived when the background pattern was potentially disruptive to the primary task in Experiment 6 (-1.358 μV) and when participants classified negative superimposed words in Experiment 7 (-.510 μV). Finally, the SPN remained when participants attended to the orientation of a diagonal line in Experiment 8 (-.589 μV). While task manipulations can turn down the extrastriate symmetry activation, they cannot render the system completely unresponsive. Permanent readiness to detect reflectional symmetry at the centre of the visual field could be an evolved adaptation.

Special Issue: “Symmetry in Cognition and Emotion”

The ten contributions of the current Special Issue on “Symmetry in Cognition and Emotion” represent different and original contributions to this topic. The new evidence spans from addressing whether the attentional blink can be elicited by internal events to the role of the fronto-parietal network. The review contributions address the effect of emotion on pseudoneglect and the role of the temporal parietal junction in processing self-related information, respectively. Four contributions provide new evidence on processing different aspects of faces, such as age, gaze, emotional expression, and their effect on response inhibition. Finally, two contributions provide novel evidence on the asymmetric preferences in decisions and on the relation between preferences for visual symmetry, respectively. Taken together, these contributions provide a new insight into the different forms of “Symmetry in Cognition and Emotion”, and we hope they can help to stimulate new research.

Color Variability Constrains Detection of Geometrically Perfect Mirror Symmetry

Symmetry in nature is a result of biological self-organization, driven by evolutionary processes. Detected by the visual systems of various species, from invertebrates to primates, symmetry determines survival relevant choice behaviors and supports adaptive function by reducing stimulus uncertainty. Symmetry also provides a major structural key to bio-inspired artificial vision and shape or movement simulations. In this psychophysical study, local variations in color covering the whole spectrum of visible wavelengths are compared to local variations in luminance contrast across an axis of geometrically perfect vertical mirror symmetry. The chromatic variations are found to delay response time to shape symmetry to a significantly larger extent than achromatic variations. This effect depends on the degree of variability, i.e., stimulus complexity. In both cases, we observe linear increase in response time as a function of local color variations across the vertical axis of symmetry. These results are directly explained by the difference in computational complexity between the two major (magnocellular vs. parvocellular) visual pathways involved in filtering the contrast (luminance vs. luminance and color) of the shapes. It is concluded that color variability across an axis of symmetry proves detrimental to the rapid detection of symmetry, and, presumably, other structural shape regularities. The results have implications for vision-inspired artificial intelligence and robotics exploiting functional principles of human vision for gesture and movement detection, or geometric shape simulation for recognition systems, where symmetry is often a critical property.