Speed, more than depth, determines the strength of induced motion

Motion of an inducer in a given direction can cause illusory motion in the opposite direction in a neighboring target object, a phenomenon called induced motion. Induced motion may be due to inducer-target interactions that are local in visual space. Accordingly, increasing the depth between inducer and target should weaken induced motion. Alternatively, separation in depth may not determine whether one object can affect the motion of another. Either viewpoint is supported by separate studies. We show that this contradiction is due to a methodological artifact related to target velocity. Our results support the suggestion that induced motion is not affected by depth separation. Participants rated the effect of an inducer on a target dot presented at different disparities. When target velocity varied with depth, induced motion decreased with depth separation. When target velocity was constant across depth, induced motion was also constant across depth. Thus, target velocity, more than depth, influence motion induction.