Different effects of flash-grab and frame stimuli on position shift and shape distortion.

In the flash-grab effect, an object flashed on a moving background appears to be shifted in the direction of the motion. The same background motion also distorts the flashed object's perceived shape. An even greater shift in the perceived location is produced by the frame effect, raising the question of whether it also produces a shape distortion. This phenomenon is important because the frame effect has been linked to perceptual stabilization during eye movements where the whole visual field acts as the frame. We found that, unlike the flash-grab case, shape was preserved for the frame effect to a much greater extent than for the flash-grab. Next, we tested the extent to which shape distortions could be predicted from the size of the shifts in position of individual shape elements. We found that observed distortions were weaker than predicted distortions for the frame effect, but stronger for the flash-grab stimulus. Finally, we examined whether the greater shape distortion for the flash-grab was due to the nature of the background motion (rotation vs. translation) or the aperture within which the background motion was presented (circular vs. rectangular). We found that both factors contributed to greater shape distortion. Our findings show that motion-induced shape distortions are not solely based on the individual position shifts of the shape elements when tested in isolation. The shape preservation for the frame effect may be achieved through engaging shape-based mechanisms tuned to the dynamics of saccadic eye movements.