Shaya S Samet^1,2 (), Jasman Kahlon^1,2, Nicholas Baker³, Erez Freud^1,2, James H Elder^1,2, Peter J Kohler^1,2; ¹York University, ²Center for Vision Research, ³Loyola University

Object shape perception depends on both local shape features, such as contour curvature, and configural shape information, which arises from the spatial relationships of these features. Specialized stimuli and behavioural methods have dissociated local and configural shape contributions. Here we used high-density EEG to investigate cortical mechanisms of local and configural shape perception, with an SSVEP paradigm that isolates differential brain responses between image-pairs. Stimuli included intact natural animal silhouettes, silhouettes where configural shape had been disrupted (‘Frankensteins’; Baker & Elder, 2022), and synthetic curvature-matched-controls where local curvature was progressively matched to natural silhouettes (CMCs; Elder et al., 2018). In Study 1 (n=32), we compared pairs of CMC classes and intact animals to assess effects of local curvature and global configural constraints. Results revealed shape-contingent activity in occipital and temporal cortices peaking ~150-250ms post-stimulus, influenced by both local and configural information. Local curvature had measurable effects on brain responses, but even when paired with CMCs with identical local curvature statistics, natural silhouettes produced robust differential responses. The latter response was subject to an inversion effect, reflecting semantic and holistic processing, but using inverted silhouettes did not fully abolish differential responses, suggesting that some configural processing survives inversion. In Study 2 (n=30), we compared Frankenstein and intact shapes for shapes where ‘Frankensteining’ had strong or weak behavioural effects on object recognition. Responses to Frankenstein stimuli were smaller overall, peaked later (~350ms), and were localized over right temporal cortex. Importantly, these responses were abolished by inversion, and were only found for stimulus pairs that produced strong behavioural effects, establishing the role of the observed brain activity in configural shape perception. These studies demonstrate that local and configural shapes produce distinct brain responses, and that our approach can isolate brain mechanisms that process configural information to support object recognition.

Acknowledgements: This work was supported by the Vision Science to Applications (VISTA) program funded by the Canada First Research Excellence Fund (CFREF 2016–2023) and by a Discovery Grant from the Natural Sciences and Engineering Research Council of Canada awarded to Peter J Kohler.