Motion-induced illusory displacement reexamined: differences between perception and action?

The position of a drifting sine-wave grating enveloped by a stationary Gaussian is misperceived in the direction of motion. Previous research indicated that the illusion was larger when observers pointed to the center of the stimulus than when they indicated the stimulus position on a ruler. This conclusion was reexamined. Observers pointed to the position of a small Gabor patch on the screen or compared its position to moving patches, stationary lines, or flashed lines. With moving patches, the illusion was larger with probe than with motor judgments; with stationary lines, the illusion was about the same size; and with flashed lines, the illusion was smaller with probe than with motor judgments. Thus, the comparison between perceptual and motor measures depended strongly on the methods used. Further, the target was mislocalized toward the fovea with motor judgments, whereas the target was displaced away from the fovea relative to line probes.