The perceived size of an object is constructed by integrating multiple sources of information, including but not limited to retinal image size, physical and perceived distance, and the relative size of different objects in a scene. Here, we introduce a novel size-contrast illusion that highlights the role of dynamic visual information in modulating the contribution of different sources of information in determining the perceived size of an object. In the Dynamic Illusory Size-Contrast (DISC) effect, the viewer perceives the size of a target bar to be shrinking when (1) it is surrounded by an expanding box and (2) 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. Using a nulling technique, we systematically explored how these dynamic factors contribute to the DISC effect. Importantly, the expanding box was necessary but not sufficient to induce an illusory percept, distinguishing the DISC effect from other size-contrast illusions. We propose that the dynamic nature of the stimulus leads to greater uncertainty regarding the retinal size of the target object. As a result, other sources of information, such as relative size, contribute more to its perceived size, thereby increasing the magnitude of the illusory percept. 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. In addition, the DISC effect is not limited to a highly specific set of stimulus parameters; we highlight the generality of the illusion using stimulus configurations of classic size-contrast illusions (e.g., the Ebbinghaus and Delboeuf illusions).
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