Breathing Square Illusion


This illusion is one of several effects in which a rigid object is shown moving behind small apertures. The object appears to change size or shape as it moves. In this case the apertures become progressively narrower and wider as a square rotates behind them, and the square appears to expand and contract rhythmically ('breathe').
The illusion reveals the operation of mid-level motion processes in the brain which draw inferences about the shape of moving surfaces by integrating information across large image areas. Synchronous events in the animation lead the visual system to infer that the movement present in different apertures belongs to a single object. When the apertures are very narrow, the motion signals are predominantly along the aperture boundaries, leading to apparent expansion and contraction of the inferred object. But when the apertures are wide the extended visible contours and corners are sufficient to code the motion of the object as rotational.
Meyer, G. E., & Dougherty, T. J. (1990). Ambiguous fluidity and rigidity and diamonds that ooze. Perception19(4), 491-496.
Shiffrar, M., & Pavel, M. (1991). Percepts of rigid motion within and across apertures. Journal of Experimental Psychology: Human Perception and Performance; Journal of Experimental Psychology: Human Perception and Performance17(3), 749-761.