Apparent Motion of a Wallpaper Illusion with Lateral Head Movement

We measured the magnitude and direction of apparent motion of the wallpaper illusion produced by a lateral head movement. The wallpaper illusion was produced by converging on a grating stimulus located between a far and a near grating stimuli placed, respectively, higher and lower than the fixated middle stimulus. The proximal size of the stimuli was held constant. Observers were asked to move their head laterally on a sliding chinrest and to report the magnitude and direction of the apparent motion of each stimulus. The independent variable was the extent (2.5, 5.0, and 7.5 cm) of the head movement. The results for five observers showed that: (a) the larger mean magnitude of apparent motion was associated with larger extent of head movement, and (b) the direction of apparent motion for the far stimulus was the same as that of the head movement, and that for the near stimulus was opposite to that of the head movement. These results are consistent with Gogel's hypothesis of an apparent concomitant motion of the object with head movement when the egocentric distance of the object is misperceived.