Using the kinetic Zollner illusion to quantify the interaction between form and motion information in depth

In the kinetic Zollner illusion a stimulus moving over a background of oriented lines appears tilted away from the line orientation. This "motion-tilt" illusion is a powerful demonstration of how form information can influence the computation of motion, particularly in signaling motion direction. In the present study, using a random dot stereogram of the kinetic Zollner illusion, we examined whether and how the degree of motion tilt is affected when form and motion components of the illusion are separated in depth. In Experiment 1 we showed that increasing the depth separation (by increasing binocular disparity) between the moving stimulus and oriented lines attenuated the motion-tilt effect. Motion tilt induction was observed for depth separations of -18 to 18 arcmin in uncrossed and crossed directions, but not at larger separations. In Experiment 2 we showed that motion tilt induction in the kinetic Zollner illusion was also observed when multiple oriented planes were presented in conjunction with a moving stimulus. However, the direction and extent of the illusory motion tilt was determined by the nearest oriented plane. Collectively, these findings show that the interaction of form and motion is dependent on depth and is optimally tuned for a small range of separations.