Abstract
Purpose: When viewing transparently superimposed parallel and ‘expanding radial' dots, observers perceive the trajectory of some radial dots to be curved when, in fact, all trajectories are straight. The conclusion from prior experiments was that this illusion was a result of induced motion and pursuit eye movements. Observers also reported seeing the illusion when asked to track a horizontally moving (HM) dot in the absence of the parallel dots. The present experiments examine whether this finding is due to a minimal background being present or to a curved trajectory being traced on the retina. This investigation also examines how the strength of the illusion varies as a function of the direction of radial dot motion and the presence of a stationary central fixation (SCF) dot. Methods: The radial dot moved in 1 of 6 directions (clockwise: 0, 45, 135, 180, 225, or 315 degrees), had a straight or curved trajectory, and was embedded into a field of leftward parallel dots or presented alone. Observers fixated on the accelerating radial dot, on the constant-velocity HM dot, on the SCF dot, or were instructed to keep their eyes in the “center” while judging the trajectory of the radial or HM dot. Results: No illusion was reported in either the minimal background or the curved-trace conditions. Observers reported less of an illusion when the radial dot's direction of motion was against the movement of the parallel dots, and reported more of an illusion when asked to keep their eyes in the “center” versus when asked to fixate on a SCF. Conclusion: Results suggest that horizontal motion may be interpreted as a stable characteristic of the environment. The findings also provide support for the argument that induced motion plays a role in the trajectory illusion. However, the results question previous findings that the perceived shift in the focus of expansion (Duffy/Wurtz illusion) is not due to eye movements, but, rather, to an eye movement compensation mechanism.
Support: NSERC (Canada) & FCAR Research Grants to MvG.