Contributions of visible persistence and perceptual set to the flash-lag effect: focusing on flash onset abolishes the illusion.

Among other theories, visible persistence has been suggested to explain the flash-lag effect (FLE). According to this account, the flash is not compared to the moving object at its perceived onset, but at a later time while it is still subjectively visible. Therefore, it is reported to lag the moving object. We show that observers' perceptual set determines whether the persisting image of the flash or its onset is used to judge relative position. Spontaneously, observers use the persisting image, such that a flash-lag results. When forced to focus on the onset of the flash because flashes and stationary onset-only stimuli are mixed, observers base their judgments on the onset and the FLE is abolished. We found that perceptual set affected the FLE in the flash-initiated and in the continuous-motion paradigm. Finally, we showed that the position of the moving object was perceived without any blur and that the flash persisted subjectively for at least 60-80ms. Changes of perceptual set and visible persistence may underlie many of the previously reported modulations of the FLE.