Luminance-polarity distribution across the symmetry axis affects the electrophysiological response to symmetry

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.

Symmetric patterns with different luminance polarity (anti‐symmetry) generate an automatic response in extrastriate cortex

“…We know anti‐symmetry generates a similar, but slightly reduced SPN (Makin et al., ; Wright, Mitchell, Dering, & Gheorghiu, , Figure b–e). Although Oka, Victor, Conte, and Yanagida () documented an odd‐harmonic steady‐state visual‐evoked potential (SSVEP) response to anti‐symmetry under passive viewing conditions, no previous work has compared SPN responses to symmetry and anti‐symmetry under different task conditions on the same participants.…”

“…This aspect of Wright et al. () also contradicts the selective attention account. Furthermore, Oka et al.…”

“…We acknowledge that there may be other forms of anti‐symmetry which are not captured by the visual system's intrinsic coding, and are only discovered indirectly through serial visual search (van der Helm & Treder, ; Mancini et al., ). However, this does not apply to the dot patterns used in recent SPN research (Makin et al., ; Wright et al., ). Element density and presence of substructures are likely to be key determinants of automatic anti‐symmetry perception.…”

“…Wright et al. () reported that symmetry and anti‐symmetry dot patterns generated a comparable sequence of stable topographic microstates, suggesting that the same sequence of perceptual and cognitive processes were recruited in both cases. This aspect of Wright et al.…”

“…Wright et al. () also considered SPNs generated by polarity‐grouped anti‐symmetry, where all elements on one side are black, and all elements on the other side are white. This generated a very similar SPN to symmetry, and a larger SPN than conventional anti‐symmetry.…”