Measuring Integration Processes in Visual Symmetry with Frequency-Tagged EEG

Abstract

Symmetry is a highly salient feature of the natural world which requires integration of visual features over space. The aim of the current work is to isolate dynamic neural correlates of symmetry-specific integration processes. We measured steady-state visual evoked potentials (SSVEP) as participants viewed symmetric patterns comprised of distinct spatial regions presented at two different frequencies (f1 and f2). We measured intermodulation components, shown to reflect non-linear processing at the neural level, indicating integration of spatially separated parts of the pattern. We generated a wallpaper pattern containing two reflection symmetry axes by tiling the plane with a two-fold reflection symmetric unit-pattern and split each unit-pattern diagonally into separate parts which could be presented at different frequencies. We compared SSVEPs measured for wallpapers and control patterns for which both images were equal in terms of translation and rotation symmetry but reflection symmetry could only emerge for the wallpaper pattern through integration of the image-pairs. We found that low-frequency intermodulation components differed between the wallpaper and control stimuli, indicating the presence of integration mechanisms specific to reflection symmetry. These results showed that spatial integration specific to symmetry perception can be isolated through a combination of stimulus design and the frequency tagging approach.

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

“…Other SPN work has shown that the extrastriate symmetry network always responds to symmetry in the image, but can sometimes go beyond this and recover symmetry in the object, independent of changes in virtual view angle ( Makin, Rampone, & Bertamini, 2015 ) and despite partial occlusion ( Rampone, Makin, Tatlidil, & Bertamini, 2019 ). Finally, the steady-state visual evoked potential approach can be used to isolate the symmetry response in the odd harmonic frequencies ( Alp, Kohler, Kogo, Wagemans, & Norcia, 2018 ; Kohler et al., 2016 ; Norcia, Candy, Pettet, Vildavski, & Tyler, 2002 ) and the amplitude of this signal also scales with symmetry salience ( Oka, Victor, Conte, & Yanagida, 2007 ).…”

Effect of change saliency and neural entrainment on flicker-induced time dilation

“…Boremanse et al (2014) reported that the beat was absent when the two halves of a face stimulus were misaligned or separated. Alp et al (2018) also revealed that conditions without global integration (i.e. scrambled control and rotated control) had smaller beats compared with those using reflection symmetry stimuli.…”

“…IM components were only specific to the holistic processing condition when two halves formed the whole face (Boremanse, Norcia, & Rossion, 2013;Boremanse, Norcia, & Rossion, 2014). IM components have also been shown to represent perceptual binding in binocular and interocular rivalry (Sutoyo & Srinivasan, 2009), in perceiving coherent patterns (Cunningham, Baker, & Peirce, 2017), illusory contours (Alp, Kogo, Van Belle, Wagemans, & Rossion, 2016), and symmetry (Alp, Kohler, Kogo, Wagemans, & Norcia, 2018). Therefore, beat as one of IM components is well-established in vision studies.…”

The neural basis of visual symmetry and its role in mid‐ and high‐level visual processing

“…In a recent follow‐up study, Alp et al . measured SSVEP as participants viewed symmetric patterns composed of distinct spatial regions presented at two different frequencies ( f 1 and f 2 ).…”