Differential effects of the rod-and-frame illusion on the timing of forearm rotations

The current study focused on the time course of the effects of the rod-and-frame illusion (RFI) on the kinematics of targeted forearm rotations. Participants were asked to reproduce perceived rod orientations by propelling a hand-held cylinder forward while rotating it to the target orientation. Rod and frame orientations were systematically varied, and cylinder rotations were normalized to time. Average realized cylinder orientations confirmed that when the frame orientation deviated from the vertical, a reproduction error occurred in the direction opposite to the direction of the frame tilt. In contrast, the perceived orientation of the stimulus rod was exaggerated relative to the vertical (i.e., reproduction errors were in the direction of the rod tilt). Furthermore, linear regression analyses for every normalized time sample showed that the rod and frame effects start simultaneously, but they reach their maximum effect at different points in time. We discuss the implications of our findings for current views on the effects of visual illusions on motor control.