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

Abstract

The flash-drag effect (FDE) refers to the phenomenon in which the position of a stationary flashed object in one location appears shifted in the direction of motion in another location in the visual field. 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 only two frames of apparent motion stimuli can preattentively cause the FDE with a broad temporal tuning. In the next experiment, we isolated two-frame apparent motion stimuli from the random-motion sequence we used previously, and measured the FDE by presenting the flash at various timings relative to the jump. Under this condition, observers could easily identify the direction of motion. We found the FDE again with a similar temporal tuning, but its amplitude was substantially larger than the effect originally found with the random-motion sequence. Overall, the results of this study suggest that the FDE involves preattentive motion processing but that top-down processes may also be involved in facilitation of the FDE.