Sustained attention and the flash grab effect.

When a stationary target is briefly presented on top of a moving background as it reverses direction, the target is displaced perceptually in the direction of the upcoming motion (the flash grab effect). To determine the role of attention in this effect, we investigated whether the predictability of the location of the flash grab target modulates the illusion. First, we established that effect was weaker for spatially predictable targets. Next, we showed that the flash grab effect decreased for a narrower spatial spread of attention before the onset of the target and that it was smaller for left hemifield presentations than right. Finally, we demonstrated that diverting attention away from the target and the background motion decreases the flash grab effect. In the first two experiments, the decrease in the illusion could be attributed to either increased attention to the target or decreased attention to the motion; we assume that increasing attention to the target necessarily decreases attention to the motion. However, in the final experiment, the central task decreases attention to both the target and the motion. The results show a decrease in the illusion and that reveals that attention to the motion is the primary causal factor.