title: Quantifying the Ebbinghaus figure effect: target size, context size, and target-context distance determine the presence and direction of the illusion.
abstract: Over the last 20 years, visual illusions, like the Ebbinghaus figure, have become widespread to investigate functional segregation of the visual system. This segregation reveals itself, so it is claimed, in the insensitivity of movement to optical illusions. This claim, however, faces contradictory results (and interpretations) in the literature. These contradictions may be due to methodological weaknesses in, and differences across studies, some of which may hide a lack of perceptual illusion effects. Indeed, despite the long history of research with the Ebbinghaus figure, standardized configurations to predict the illusion effect are missing. Here, we present a complete geometrical description of the Ebbinghaus figure with three target sizes compatible with Fitts' task. Each trial consisted of a stimulus and an isolated probe. The probe was controlled by the participant's response through a staircase procedure. The participant was asked whether the probe or target appeared bigger. The factors target size, context size, target-context distance, and a control condition resulted in a 3 × 3 × 3+3 factorial design. The results indicate that the illusion magnitude, the perceptual distinctiveness, and the response time depend on the context size, distance, and especially, target size. In 33% of the factor combinations there was no illusion effect. The illusion magnitude ranged from zero to (exceptionally) 10% of the target size. The small (or absent) illusion effects on perception and its possible influence on motor tasks might have been overlooked or misinterpreted in previous studies. Our results provide a basis for the application of the Ebbinghaus figure in psychophysical and motor control studies.
authors.affiliation: Institut de Neurosciences des Systèmes, Aix-Marseille Université Marseille, France ; Institut de la Santé et de la Recherche Médical, UMR_S 1106 Marseille, France ; Systems Control and Flight Dynamics Department, Office National d'Etudes et de Recherches Aérospatiales (ONERA) Salon de Provence, France.
authors.forename: Hester
authors.initials: H
authors.lastname: Knol
authors.affiliation: Centre National de la Recherche Scientifique Paris, France ; Centre de Recherche Cerveau & Cognition - UMR5549, Université Toulouse III - Paul Sabatier Toulouse, France.
authors.forename: Raoul
authors.initials: R
authors.lastname: Huys
authors.affiliation: Systems Control and Flight Dynamics Department, Office National d'Etudes et de Recherches Aérospatiales (ONERA) Salon de Provence, France.
authors.forename: Jean-Christophe
authors.initials: JC
authors.lastname: Sarrazin
authors.affiliation: Institut de Neurosciences des Systèmes, Aix-Marseille Université Marseille, France ; Institut de la Santé et de la Recherche Médical, UMR_S 1106 Marseille, France ; Centre National de la Recherche Scientifique Paris, France.
authors.forename: Viktor K
authors.initials: VK
authors.lastname: Jirsa
keywords: Ebbinghaus illusion; dorsal stream; perception; ventral stream; visual illusions