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Abstract
Geometrical visual illusions are an intriguing phenomenon, in which subjective perception consistently misjudges the objective, physical properties of the visual stimulus. Prominent theoretical proposals have been advanced attempting to find common mechanisms across illusions. But empirically testing the similarity between illusions has been notoriously difficult because illusions have very different visual appearances. Here we overcome this difficulty by capitalizing on the variability of the illusory magnitude across participants. Fifty-nine healthy volunteers participated in the study that included measurement of individual illusion magnitude and structural MRI scanning. We tested the Muller-Lyer, Ebbinghaus, Ponzo, and vertical-horizontal geometrical illusions as well as a non-geometrical, contrast illusion. We found some degree of similarity in behavioral judgments of all tested geometrical illusions, but not between geometrical illusions and non-geometrical, contrast illusion. The highest similarity was found between Ebbinghaus and Muller-Lyer geometrical illusions. Furthermore, the magnitude of all geometrical illusions, and particularly the Ebbinghaus and Muller-Lyer illusions, correlated with local gray matter density in the parahippocampal cortex, but not in other brain areas. Our findings suggest that visuospatial integration and scene construction processes might partly mediate individual differences in geometric illusory perception. Overall, these findings contribute to a better understanding of the mechanisms behind geometrical illusions.
Highlights
Participant and illusion we established a participant’s individual illusion magnitude
While the research of geometrical illusions of recent years has mostly focused on the role of low-level visual processing, here we asked whether more high-level, visuospatial mechanisms might play a role in the misjudgment of geometrical illusions
To be able to compare the results across geometrical illusions, for each illusion we computed the illusion magnitude, which was a log transformed ratio of subjective perceptual estimation vs. objective, real stimulus length or brightness
Summary
Participant and illusion we established a participant’s individual illusion magnitude. To examine the potential neural mechanisms underlying geometrical illusions we correlated between the behavioral illusion magnitude and the gray matter density in the brain (voxel-based morphometry analysis[14]). The benefit of this approach is that in contrast to most previously used fMRI/ERP designs in our approach only the actual illusion effect is compared across participants, permitting us to elucidate the neural underpinnings of the illusions in a more specific way. Finding a correlation between gray matter density in the PHC/hippocampus and magnitude of geometrical illusion will support the view that visuospatial integration and scene construction process might play a role in perception of geometrical illusions. To investigate the regional specificity of the PHC/hippocampus, we examined a number of control regions that have been implicated in scene and visual processing in the temporal, occipital, and parietal lobes, including the retrosplenial cortex (RSC) and the transverse occipital sulcus (TOS) scene-selective regions
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