This demonstration shows how movement can influence perceived position. The upper two patches of grating lie directly above the lower two patches, but the movement within each patch causes them to appear misaligned. Each patch appears shifted in position towards the direction of its movement, so the upper patches appear closer together than the lower patches. Use the playback controls to pause the animation and verify that the patches are really aligned.
The effect is called 'motion-induced position shift', and was first reported by Ramachandran & Anstis (1990). Several explanations have been proposed. One argues that the shift reflects the visual system's anticipation of the future position of the patches (Ramachandran & Anstis, 1990). Another proposes that it is due to apparent enhancement of contrast at the leading motion edge and diminution of contrast at the trailing edge (Arnold et al., 2007). However as yet no explanation can offer a full account of the experimental observations.
See:
Arnold D. H., Thompson M., Johnston A. (2007). Motion and position coding. Vision Research, 47, 2403–2410.
Mather, G., Pavan, A. (2009) Motion-induced position shifts occur after motion integration. Vision Research, 49, 2741-2746. PDF
Ramachandran V. S., Anstis S. M. (1990). Illusory displacement of equiluminous kinetic edges. Perception, 19, 611–616.
Rider, A. T., McOwan, P. W., & Johnston, A. (2009). Motion-induced position shifts in global dynamic Gabor arrays. Journal of Vision, 9(13).