Backscroll illusion is an apparent motion perceived in backgrounds of movie images that present locomotive objects such as people, animals, and vehicles. This illusion is from the visual system registering retinal motion signals in relation to high-level object motion signals. We confirmed this notion from psychophysical experiments that mainly presented a realistic human figure on a treadmill walking or running in front of a counterphase grating. The apparent grating motion was consistently induced in the direction opposite to the locomotion. The induction was tuned to a gait velocity. The time course showed that the illusion arose as if it was synchronized with gait recognition, and that it was sustained against several reversals of limb swings so that local motion accounts were denied. A weak but significant illusion was observed from a static figure that implied a gait. Thus, we concluded that the illusion was determined by the high-level recognition of biological motion. An additional experiment found a similar effect from a vehicle with rotating wheels but no induction from a rotating wheel per se. This result led us to hypothesize that the backscroll illusion is generalized to objects that have shapes implying their moving directions.
