The Ebbinghaus illusion is a classic example of the influence of a contextual surround on the perceived size of an object. Here, we introduce a novel variant of this illusion called the Dynamic Ebbinghaus illusion in which the size and eccentricity of the surrounding inducers modulates dynamically over time. Under these conditions, the size of the central circle is perceived to change in opposition with the size of the inducers. Interestingly, this illusory effect is relatively weak when participants are fixating a stationary central target, less than half the magnitude of the classic static illusion. However, when the entire stimulus translates in space requiring a smooth pursuit eye movement to track the target, the illusory effect is greatly enhanced, almost twice the magnitude of the classic static illusion. A variety of manipulations including target motion, peripheral viewing, and smooth pursuit eye movements all lead to dramatic illusory effects, with the largest effect nearly four times the strength of the classic static illusion. We interpret these results in light of the fact that motion-related manipulations lead to uncertainty in the image size representation of the target, specifically due to added noise at the level of the retinal input. We propose that the neural circuits integrating visual cues for size perception, such as retinal image size, perceived distance, and various contextual factors, weight each cue according to the level of noise or uncertainty in their neural representation. Thus, more weight is given to the influence of contextual information in deriving perceived size in the presence of stimulus and eye motion. Biologically plausible models of size perception should be able to account for the reweighting of different visual cues under varying levels of certainty.

We recently showed that motion dynamics greatly enhance the magnitude of certain size contrast illusions, such as the Ebbinghaus and Delboeuf illusions. Here, we extend our study of the effect of motion dynamics on size illusions through a novel dynamic corridor illusion, in which a single target translates along a corridor background. Across three psychophysical experiments, we quantify the effects of stimulus dynamics on the Ebbinghaus and corridor illusions across different viewing conditions. The results revealed that stimulus dynamics had opposite effects on these different classes of size illusions. Whereas dynamic motion enhanced the magnitude of the Ebbinghaus illusion, it attenuated the magnitude the corridor illusion. Our results highlight precision-driven weighting of visual cues by neural circuits computing perceived object size. This hypothesis is consistent with observations beyond size perception and may represent a more general principle of cue integration in the visual system. Keywords Visual perception . Perceptual organization . Bayesian modeling An object's perceived size arises from the interaction and integration of the angular size of the object (as projected on the retina) with numerous contextual cues. Most relevant to the current study, these cues include physical and perceived dis-

We present a novel size-contrast illusion that depends on the dynamic nature of the stimulus. In the dynamic illusory size-contrast (DISC) effect, the viewer perceives the size of a target bar to be shrinking when it is surrounded by an expanding box and when there are additional dynamic cues such as eye movements, changes in retinal eccentricity of the bar, or changes in the spatial position of the bar. Importantly, the expanding box was necessary but not sufficient to obtain an illusory percept, distinguishing the DISC effect from other size-contrast illusions. We propose that the visual system is weighting the different sources of information that contribute to size perception based on the level of uncertainty in the retinal image size of the object. Whereas the growing box normally has a weak influence on the perceived size of the target bar, this influence is enhanced when other dynamic changes in the environment (e.g., eye movements, changes in retinal eccentricity, and target motion) lead to uncertainty in the retinal size of the target bar. Given the compelling nature of the DISC effect and the inherently dynamic nature of our environment, these factors are likely to play an important role in everyday size judgments.

The Ebbinghaus and Delboeuf illusions are two well-established size illusions, both of which demonstrate that the perceived size of an object depends on the physical size of the object relative to surrounding objects. This chapter reviews some of the primary observations and interpretations of these two classic illusions and some of the current theories regarding size contrast (objects appearing more different than they really are) and size assimilation (objects appearing more similar than they really are). Despite over a century of progress in visual psychology and neuroscience, many of the challenges in explaining assimilation and contrast effects on size perception remain highly relevant. Although unlikely to be due to the muscular effort of the eyes as posited by Delboeuf almost 150 years ago, the precise mechanisms that underlie size contrast and size assimilation remain unknown to this day. New discoveries about these and related effects are still being uncovered.

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