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Nihan Alp, Peter Kohler, Naoki Kogo, Johan Wagemans, Anthony Norcia; Measuring Integration Processes in Visual Symmetry with Frequency-tagged EEG. Journal of Vision 2017;17(10):175. doi: 10.1167/17.10.175.
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Previous brain imaging studies of symmetry have shown that several higher-tier visual areas have strong responses to mirror (Sasaki et al. 2005) and rotation symmetry (Kohler et al., 2016). The aim of the current work was to isolate dynamic signatures of brain responses associated with the integrative processes that underlie symmetry perception. We measured steady-state VEPs as participants viewed symmetric patterns comprised of distinct spatial regions presented a two different frequencies (f1/f2). Under these circumstances, the intermodulation (IM) components have been shown to capture integrative processing (Alp et al., 2016), because only neuronal populations that non-linearly integrate the parts of the image can produce these IMs. To measure integration processing during mirror symmetry perception, we used wallpaper patterns (Fedorov, 1891). For the mirror symmetric stimuli, we generated a PMM pattern containing two mirror symmetry axes by tiling the plane with a two-fold mirror symmetric lattice. We then diagonally split each lattice into separate parts to generate an image-pair that could be presented at different frequencies. To generate the control stimuli, we created a control pattern by rotating the first lattice by 90o, and then combining diagonally split images from the mismatched patterns. This procedure removes all mirror symmetry from the control image, while keeping local properties equal. All images contained translation and rotation symmetry, but mirror symmetry could only emerge through the combination of the image-pair in the mirror symmetric stimulus. Both mirror and control stimuli evoked activity at the IMs, indicating that non-linear integration is occurring for both pattern types. Several response components showed differential responses between the mirror and control stimuli, however, indicating symmetry-specific integration. There was a complex pattern of statistically reliable differences in both self-terms (2f1,2f2) and IMs (f2-f1), which suggests the involvement of distinctive non-linear global pooling in the presence of mirror symmetry.
Meeting abstract presented at VSS 2017
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