Object motion continuity and the flash-lag effect

When a visual object is briefly flashed, it appears to lag behind another moving object (flash-lag effect; FLE). Previous studies showed that a sudden change to the moving object at the time of the flash presentation would eliminate the FLE. We examined whether the FLE would be eliminated when a sudden color change was embedded in a sequence of color alternations on a moving object. Observers viewed a moving disc, the color of which did not change at all, changed only once when another stationary object flashed, or alternated regularly (Experiment 1) or randomly (Experiment 2) between two colors as it was moving before the flash presentation. Although the magnitude was reduced compared with the no-change condition, the FLE observed with the moving object that changed color during motion was significantly stronger than that in the one-change condition. In Experiment 3, the object color alternated between two but unexpectedly changed to a new color when the flash appeared. The elimination of FLE in such condition was comparable to the one-change condition, suggesting that the unexpected change restored the salience of the moving object at the time of flash presentation, which spared the observer from perceiving the FLE. We propose that, without an unexpected event, rapid changes in the surface feature of the moving object partially degrade the maintenance of object file, but this does not preclude the visual system in registering the existence of only one object in the motion stream.