Flash grab effect within the regions of modal and amodal completions.

When a rotating grating reverses its direction and is accompanied by a briefly flashed stimulus on top, the flash's apparent position shifts in direction after the reversal. This phenomenon, termed the flash-grab effect, can induce an illusory position shift of several degrees of visual angle, prompting investigation into scenarios in which the expected position coincides with another visual event. We investigated two such situations: perceptual filling-in at the blind spot and amodal completion behind a visible occluder. By inducing a position shift in the flash presented just outside such completed patterns, we measured the perceived angular position of the flash in the perceptual matching paradigm. We found subjective localization within the completed region of the moving inducer. Consistent results were found even when the flash was presented at a less optimal time for the flash-grab effect. Illusion size had a certain dependency on stimulus configuration, suggesting that various sources of spatial referencing are involved in the position processing around the blind-spot/occluder region. These findings imply that the visual system does not necessarily avoid a region that is devoid of physical motion stimuli when determining perceived flash position, reaching a consistent perceptual solution that integrates the motion-induced position shift and perceptual completion.