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.
Highlights
Optical illusions evoke a perceived image, color, contrast, lightness, brightness, or size that differs from the physical “reality” of the figure
For the statistical analysis the control perceptual threshold (PTcontrol) per target size was subtracted from the perceptual threshold of each trial (PTtrial) to control for the participants’ ability to judge targets of different sizes
Significant main effects for illusion magnitude were found for context size [F(2, 22) = 40.698, p = 0.000, ηp2 = 0.787], distance [F(2, 22) = 24.181, p = 0.000, ηp2 = 0.687] and target size [F(1.244,13.686) = 28.973, p = 0.000, ηp2 = 0.725]
Summary
Optical illusions evoke a perceived image, color, contrast, lightness, brightness, or size that differs from the physical “reality” of the figure. Principle factors that have been identified are the size of the target (a in Figure 1A), the context circle size (c in Figure 1A), the number of context circles (Massaro and Anderson, 1971; Roberts et al, 2005), the target-context distance (b in Figure 1A; Roberts et al, 2005; Im and Chong, 2009) and the size of the area of empty space between the context circles (Nemati, 2009) These proposed rules do not specify the exact interplay between the three parameters specified, which makes utilization of these rules for parameter selection and the prediction of the corresponding illusion effect tricky if not impossible. Franz and Gegenfurtner (2008) concluded their review stating that: “. . . currently not much is known on the exact sources of the Ebbinghaus illusion.”
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