Results of experiment 1. Psychometric curves indicate the mean fit of the data averaged across all participants. The inset of the figure illustrates the mean points of subjective equality (PSEs) for the test and reference stimuli. The black curve indicates that participants were accurately judging the radius of an unbound array. The grey curve shows that the array radius must increase by ~0.18° to be perceived as being the same size as an unbound array. Thick curves with error bars indicate the mean response across participants for each array radius. Thin curves indicate the function fitted to the data. Error bars represent standard error of the mean PSE computed across subject.  

Results of experiment 1. Psychometric curves indicate the mean fit of the data averaged across all participants. The inset of the figure illustrates the mean points of subjective equality (PSEs) for the test and reference stimuli. The black curve indicates that participants were accurately judging the radius of an unbound array. The grey curve shows that the array radius must increase by ~0.18° to be perceived as being the same size as an unbound array. Thick curves with error bars indicate the mean response across participants for each array radius. Thin curves indicate the function fitted to the data. Error bars represent standard error of the mean PSE computed across subject.  

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Our perception of an object’s size arises from the integration of multiple sources of visual information including retinal size, perceived distance and its size relative to other objects in the visual field. This constructive process is revealed through a number of classic size illusions such as the Delboeuf Illusion, the Ebbinghaus Illusion and ot...

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... Due to the contradictory results of various methods to quantify the illusion effect, and due to the large number of Ebbinghaus figure configurations tested in this study, the widely studied and applied two-up, onedown staircase procedure was chosen, which is a two alternative forced choice method (2AFC). Several previous studies also applied the staircase procedure to study different features of the Ebbinghaus figure(Roberts et al., 2005;Im and Chong, 2009;McCarthy et al., 2013). Another version of the 2AFC method to study perception is the method of constant stimuli, in which a fixed number of combinations of (Ebbinghaus) figures are shown a certain number of times in a random order. ...
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The influential two-visual streams hypothesis ascribes specific functional roles to the ventral and the dorsal network of the visual system. The ventral system has been hypothesized to process information for conscious perception (vision-forperception), whereas the dorsal stream processes information for action (vision-foraction).The idea of two separate visual networks in the human brain inspired an enormous amount of research over the past 20 or so years. The results are conflicting and divisive about the idea, causing a seemingly insurmountable gap between supporters and opponents. This thesis aims to unravel a part of the jigsaw puzzle of how perception and action are functioning.The Ebbinghaus figure consists of an object embedded in a specific context (e.g., centre circle surrounded by smaller or bigger circles). The perceived object size often deviates from its physical size, which is the so-called Ebbinghaus illusion. The Ebbinghaus figure has been used to distinguish vision-for-perception that is susceptible to visual illusions (i.e., relative size) from vision-for-action that remain unaffected by perceptions of relative sizes (i.e., absolute physical size). Albeit several papers report that the Ebbinghaus illusion affects solely perception, a growing number of studies demonstrate that action is similarly affected by this illusion. A rule to control the size perception of the centre object in the Ebbinghaus figure to ‘appear smaller’ or to ‘appear bigger’ does not exist so that predicting illusion magnitudes remains guesswork. Therefore, it remains also questionable whether all Ebbinghaus figures evoke an illusion, and which factors are key for illusion effects. We quantified the Ebbinghaus figure based on its geometry and systematically assessed its size illusion. One third of all Ebbinghaus configurations did not result in significant illusion effects. For the other part, the illusion effects were due to all geometrical parameters of the Ebbinghaus figure.After the quantification of Ebbinghaus figures, a visuomotor task was implemented in which precision and speed of the voluntary movement were investigated. The visuomotor behaviour was quantitatively and qualitatively described for discrete v and reciprocal sliding movements in terms of kinematics and the underlying dynamics. The description of the visuomotor task and of the perception of Ebbinghaus figures lead to combine both visuomotor task and Ebbinghaus figures.The latter study demonstrates that the Ebbinghaus figure influences the movement.The Ebbinghaus factors that affected perception, however, did not all appear to significantly influence the movement.Due to its systematic approach and the methodological contributions, this work can serve as a basis for future studies on the perception and action mechanisms. This dissertation demonstrated that the ventral stream and dorsal stream are not strictly functionally distinct, and that potentially different informational variables are used for ‘vision for perception’ and ‘vision for action’ irrespective of whether certain variables cause (perceptual) illusions.
... All the visual illusions employed in the Happé (1996) study are clearly characterized by global information that produces illusory percepts. The Ebbinghaus illusion (Titchener, 1901;Ebbinghaus, 1902, see Table 1 for details) produced a large number of papers (e.g., Weintraub, 1979;McCarthy et al., 2013), showing how relevant this pattern is in vision sciences. ...
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A visual illusion refers to a percept that is different in some aspect from the physical stimulus. Illusions are a powerful non-invasive tool for understanding the neurobiology of vision, telling us, indirectly, how the brain processes visual stimuli. There are some neurodevelopmental disorders characterized by visual deficits. Surprisingly, just a few studies investigated illusory perception in clinical populations. Our aim is to review the literature supporting a possible role for visual illusions in helping us understand the visual deficits in developmental dyslexia and autism spectrum disorder. Future studies could develop new tools - based on visual illusions - to identify an early risk for neurodevelopmental disorders.
... Due to the contradictory results of various methods to quantify the illusion effect, and due to the large number of Ebbinghaus figure configurations tested in this study, the widely studied and applied two-up, onedown staircase procedure was chosen, which is a two alternative forced choice method (2AFC). Several previous studies also applied the staircase procedure to study different features of the Ebbinghaus figure (Roberts et al., 2005;Im and Chong, 2009;McCarthy et al., 2013). Another version of the 2AFC method to study perception is the method of constant stimuli, in which a fixed number of combinations of (Ebbinghaus) figures are shown a certain number of times in a random order. ...
... A big area of uncertainty might be linked to a shallow slope of the psychometric function, and the PT should be equal to the point of subjective equality. McCarthy et al. (2013) have performed 4 experiments with using both the staircase procedure (experiment 2) and the method of constant stimuli (experiments 1, 3, and 4) showing that both methods result in similar points of subjective equality. Considering the long history of staircase procedures in the field of psychophysics (García-Pérez, 1998), and the magnitude of the illusion effect being in a similar range as in the similar study of Roberts et al. (2005) the staircase procedure opens new doors in order to quantify the Ebbinghaus illusion effect in a systematic way. ...
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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) ten percent 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.
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