title: Robust size illusion produced by expanding and contracting flow fields.
abstract: A new illusion is described. Randomly positioned dots moved radially within an imaginary annular window. The dots' motion periodically changed the direction, leading to an alternating percept of expanding and contracting motion. Strikingly, the apparent size of the enclosed circular region shrank during the dots' expanding phases and dilated during the contracting phases. We quantitatively measured the illusion, and found that the presence of energy at the local kinetic edge could not account for the illusion. Besides, we reproduced the illusion on a natural scene background seen from a first-person point of view that moved forward and backward periodically. Blurring the boundaries of motion areas could not reverse the illusion in all subjects. Taken together, our observed illusion is likely induced by optic flow processing with some components of motion contrast. Expanding or contracting dots may induce the self-motion perception of either approaching or leaving way from the circle. These will make the circle appear smaller or larger since its retinal size remains constant.
authors.affiliation: CAS Key Laboratory of Behavioral Science, Institute of Psychology, Beijing, PR China; Department of Psychology, University of Chinese Academy of Sciences, PR China.
authors.forename: Xue
authors.initials: X
authors.lastname: Dong
authors.affiliation: CAS Key Laboratory of Behavioral Science, Institute of Psychology, Beijing, PR China.
authors.forename: Jianying
authors.initials: J
authors.lastname: Bai
authors.affiliation: CAS Key Laboratory of Behavioral Science, Institute of Psychology, Beijing, PR China. Electronic address: baom@psych.ac.cn.
authors.forename: Min
authors.initials: M
authors.lastname: Bao
mesh_terms: D000328:Adult; D000704:Analysis of Variance; D003867:Depth Perception / Q000502:physiology*; D005260:Female; D006801:Humans; D008297:Male; D009039:Motion Perception / Q000502:physiology*; D009903:Optical Illusions / Q000502:physiology*; D010775:Photic Stimulation / Q000379:methods; D012858:Size Perception / Q000502:physiology*; D055815:Young Adult
keywords: Depth perception; Motion; Optic flow; Size constancy; Size illusion