Abstract
Electrophysiological studies of symmetry have found a difference wave termed the Sustained Posterior Negativity (SPN) related to the presence of symmetry. Yet the extent to which the SPN is modulated by luminance-polarity and colour content is unknown. Here we examine how luminance-polarity distribution across the symmetry axis, grouping by luminance polarity, and the number of colours in the stimuli, modulate the SPN. Stimuli were dot patterns arranged either symmetrically or quasi-randomly. There were several arrangements: ’segregated’-symmetric dots were of one polarity and randomly-positioned dots were of the other; ‘unsegregated’-symmetric dots were of both polarities in equal proportions; ‘anti-symmetric’-dots were of opposite polarity across the symmetry axis; ‘polarity-grouped anti-symmetric’-this is the same as anti-symmetric but with half the pattern of one polarity and the other half of opposite polarity; multi-colour symmetric patterns made of two, three to four colours. We found that the SPN is: (i) reduced by the amount of position-symmetry, (ii) sensitive to luminance-polarity mismatch across the symmetry axis, and (iii) not modulated by the number of colours in the stimuli. Our results show that the sustained nature of the SPN coincides with the late onset of a topographic microstate sensitive to symmetry. These findings emphasise the importance of not only position symmetry, but also luminance polarity matching across the symmetry axis.
Highlights
Symmetry, a ubiquitous feature in natural scenes, is found in both biological and man-made objects, and is detected effortlessly by the human visual system
We examined across three experiments how luminance-polarity distribution across the symmetry axis, grouping by luminance-polarity in anti-symmetric and symmetric patterns, and the number of colours in the stimuli affect symmetry detection and the Sustained Posterior Negativity (SPN) response to symmetry
We found: (A) superior performance for luminance-polarity-grouped compared to classic anti-symmetric patterns suggesting that symmetry perception in anti-symmetric patterns can benefit from grouping by luminance polarity in both 50% and 100% position-symmetric conditions. (B) superior performance for the segregated and unsegregated condition in comparison to both types of anti-symmetric patterns confirming previous findings that symmetry is sensitive to luminance polarity correlations across the symmetry axis for both 50% and 100% position-symmetric patterns. (C) In contrast to behavioural performance, the SPNs obtained with 100% position-symmetric unsegregated, anti-symmetric and polarity-grouped anti-symmetric patterns were comparable in amplitude
Summary
A ubiquitous feature in natural scenes, is found in both biological and man-made objects, and is detected effortlessly by the human visual system. While recent psychophysical (behavioural) studies have examined the role of luminance-polarity and colour (Gheorghiu et al, 2016; Morales and Pashler, 1999; Wu and Chen, 2014, 2017) in symmetry perception, very little is known about how these visual attributes influence the electrophysiological correlates of symmetry perception. In this communication, we use ERP methods to investigate how luminance-polarity distribution across the symmetry axis, grouping by luminance-polarity, and the number of colours in the stimuli affect the specific SPN response to symmetry
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