Jacky Yueh¹, Joan Danielle K. Ongchoco¹; ¹University of British Columbia

We perceive not only objects and motions but also the relationships between them—as in the perception of causality. But causal interactions occur not only in simple collisions, but also in complex causal systems, such as pulleys and gears. These examples are typically associated with effortful and abstract reasoning, but here, in contrast, we introduce a “rotating gears” illusion in which sustained causal dynamics seem to reshape basic visual properties. When observers fixate on one of two rotating gears (with directly aligned teeth), the adjacent gear will appear to rotate faster even though they have the same speed, due to greater motion sensitivity in peripheral vision. But when equidistant gears are properly offset so their teeth slot nicely in between each other (making them *causally enmeshed*), the adjacent gear seems to rotate even faster. We verified this experimentally: Observers saw pairs of gears and reported how much faster the adjacent gear rotated relative to the one fixated on. Speed estimates were greater for causally-enmeshed gears versus not. These were reliably different from estimates in critical control conditions, such as when observers fixated at the center (so we shouldn’t expect an illusion of speeding-up) and when observers saw “poker chip”-like discs whose edges matched the alternating black and white gear teeth and notches but could not be causally enmeshed (isolating causal connection from mere alternating black and white motion). Causal enmeshment also generates an entirely different illusion when we modify the transparency of the adjacent gear. In an independent experiment, we verified that a semi-transparent adjacent gear appears more visible when causally enmeshed with the gear that is fixated on, compared to when it is not. Causal enmeshment can sharpen visual properties of speed and contrast, suggesting that principles of more complex causal dynamics may be more deeply embedded in visual processing.