A flash-drag effect in random motion reveals involvement of preattentive motion processing

The flash-drag (FDE) effect refers to the phenomenon in which the position of a stationary flashed object in one location appears shifted in the direction of nearby motion. Over the past decade, it has been debated how bottom-up and top-down processes contribute to this illusion. In this study, we demonstrate that randomly phase-shifting gratings can produce the FDE. In the random motion sequence we used, the FDE inducer (a sinusoidal grating) jumped to a random phase every 125 ms and stood still until the next jump. Because this random sequence could not be tracked attentively, it was impossible for the observer to discern the jump direction at the time of the flash. By sorting the data based on the flash's onset time relative to each jump time in the random motion sequence, we found that a large FDE with a broad temporal tuning occurred around 50 to 150 ms before the jump and that this effect was not correlated with any other jumps in the past or future. These results suggest that as few as two frames of unpredictable apparent motion can preattentively cause the FDE with a broad temporal tuning.