Rotating Snakes Illusion-Quantitative Analysis Reveals a Region in Luminance Space With Opposite Illusory Rotation.

Lea Atala-Gérard,Michael Bach

doi:10.1177/2041669517691779

Abstract

The Rotating Snakes Illusion employs patterns with repetitive asymmetric luminance steps forming a “snake wheel.” In the underlying luminance sequence {black, dark grey, white, light grey}, coded as {0, g1, 100, g2}, we varied g1 and g2 and measured illusion strength via nulling: Saccades were performed next to a “snake wheel” that rotated physically; observers adjusted rotation until a stationary percept obtained. Observers performed the perceptual nulling of the seeming rotation reliably. Typical settings for (g1, g2), measured from images by Kitaoka, are around (20%, 60%). Indeed, we found a marked illusion in the region (g1≈{0%–25%}, g2≈{20%–75%}) with a rotation speed of ≈1°/s. Surprisingly, we detected a second “island” around (70%, 95%) with opposite direction of the illusory rotation and weaker illusion. Our quantitative measurements of illusion strength confirmed the optimal luminance choices of the standard snake wheel and, unexpectedly, revealed an opposite rotation illusion.

Highlights

Faubert and Herbert (1999) used very similar patterns and called it the ‘‘peripheral drift illusion.’’ These patterns and their colours were optimised by Kitaoka (2003), resulting in the strong and widely known ‘‘Rotating Snakes Illusion.’’ In 2014, Kitaoka called it ‘‘Fraser-Wilcox illusion’’ (Kitaoka, 2014), but we will use the term ‘‘Rotating Snakes Illusion’’ throughout this article
Our observers found the nulling procedure intuitive and convincing. When they looked directly at the illusion test disk after the trial, they were surprised by its physical rotation
An opposite illusory rotation obtained in another region of the luminance space, (g1 & 70%, g2 & 95%; blueish)

Summary

Introduction

Faubert and Herbert (1999) used very similar patterns and called it the ‘‘peripheral drift illusion.’’ These patterns and their colours were optimised by Kitaoka (2003), resulting in the strong and widely known ‘‘Rotating Snakes Illusion.’’ In 2014, Kitaoka called it ‘‘Fraser-Wilcox illusion’’ (Kitaoka, 2014), but we will use the term ‘‘Rotating Snakes Illusion’’ throughout this article. I-Perception be stronger when rendered in colour (Backus & Oruc , 2005). Four explanatory models exist (Backus & Oruc , 2005; Conway, Kitaoka, Yazdanbakhsh, Pack, & Livingstone, 2005; Fermuller, Ji, & Kitaoka, 2010; Murakami, Kitaoka, & Ashida, 2006). To further our understanding of the underlying mechanisms, we undertook a quantitative analysis of how the strength of the illusion depends on the luminance levels in the repetitive pattern

Methods

Results

Discussion

Conclusion