Perceptual similarity and the neural correlates of geometrical illusions in human brain structure

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.

Factors underlying visual illusions are illusion-specific but not feature-specific

“…Interestingly, we also found very weak correlations-except for one-between the magnitudes of visual illusions (Grzeczkowski, Clarke, Francis, Mast, & Herzog, 2017; see also Axelrod, Schwarzkopf, Gilaie-Dotan, & Rees, 2017). Patients with schizophrenia similarly showed only weak correlations between different illusion magnitudes (Grzeczkowski et al, 2018; see also Kaliuzhna et al, 2018).…”

Dynamic presentation boosts the Ebbinghaus illusion but reduces the Müller-Lyer and orientation contrast illusions

“…The correlations across participants for the strengths of visual illusions are typically very weak (e.g., Axelrod, Schwarzkopf, Gilaie-Dotan & Rees, 2017, Grzeczkowski, Clarke, Francis, Mast & Herzog, 2017.…”

Individual differences in visual science: What can be learned and what is good experimental practice?

“…For example, Duncan and Boynton (2003), found that the cortical magnification factor in primary visual cortex correlates with visual acuity. Recent work (see Kanai & Rees, 2011 for a review) has revealed correlations between brain structure and biological motion detection (Gilaie-dotan, Kanai, Bahrami, Rees, & Saygin, 2013), rate of perceptual alternation in binocular rivalry (Kanai, Bahrami, & Rees, 2010), motion assimilation and contrast (Takeuchi, Yoshimoto, Shimada, Kochiyama, & Kondo, 2017), susceptibility to geometrical illusions (Axelrod, Schwarzkopf, Gilaie-dotan, & Rees, 2017; Schwarzkopf, Song, & Rees, 2011), susceptibility to the tilt illusion (Song, Schwarzkopf, & Rees, 2013b; Song et al, 2013a), and orientation discrimination (Song et al, 2013b). …”