There and back again: Revisiting the on-time effect

• We examine the effect of stimulus on-time duration on multi-item apparent motion.
• Longer exposure reduced perceived motion speed under various conditions.
• We show that the on-time illusion can inform models of motion perception.

In apparent motion, static stimuli presented successively in shifted locations produce a subjective percept of continuous motion. Reducing stimulus exposure (or on-time) was shown to consistently increase the perceived velocity of apparent motion (Vision Research 29 (1989), 335–347), yet surprisingly little investigation has followed up on the discovery of this illusion. In five experiments, we delineate the boundary conditions of the on-time illusion in order to clarify its underlying mechanisms. Subjects viewed multi-item apparent-motion displays, in which at some point, on-time duration either increased or decreased. Objective velocity remained unchanged, yet participants had to judge whether they perceived the motion to become slower or faster. We observed the on-time illusion during both fast and slow apparent motion. The effect was not modulated by stimulus luminance, thus precluding an energy-summation account of the illusion. It generalized from speed perception to time perception in a temporal bisection task. The illusion was specific to apparent motion, as it did not occur with veridical motion. Finally, the illusion persisted when on-time and off-time were not confounded, that is, when off-time remained constant. These findings are discussed in the framework of current models of motion perception.