Abstract
When an object is surrounded by a moving frame or background, an illusory motion is induced into the object in the direction opposite to the motion of the reference frame (e.g. Duncker, 1929). For instance, a target moving vertically inside a horizontally moving frame appears to move obliquely (induced motion; Wallach, et al., 1978). Here we investigated whether illusory motion can act as a reference, specifically, whether the effect of the frame depends on its physical or its perceived path. To this end, we used the double-drift stimulus (Lisi & Cavanagh, 2015; aka, curveball illusion: Shapiro et al., 2010; infinite regress illusion: Hsieh and Tse, 2006) which provides an illusory direction of motion that can deviate from its veridical path by as much as ~50 dva (Lisi and Cavanagh, 2015). We presented 4 Gabors that formed the corners of a virtual square, all with vertical external motion and all with the same, horizontal direction of internal motion. When viewed in the periphery, the double-drift “square” appeared to move on an illusory oblique path. A probe spot with no internal drift was also presented inside the frame moving vertically in phase with the double-drifts’ external motion. Participants reported that the probe’s motion appeared to be oblique in the direction opposite to the frame’s illusory direction. In a second condition, the internal drift component of the double-drift stimuli was turned off and the illusory direction was extinguished. In this case, the probe followed its physical (vertical) path. In conclusion, the effect of the moving frame is determined by its consciously experienced motion rather than its physical motion.