The table below illustrates the illusory motion direction perceived in missing-fundamental gratings that move in quarter-period steps (as opposed to the veridical motion direction perceived in similar square-wave and sine-wave gratings). [If you don't see animation in the bottom row of the table, then you'll need an updated browser.]

Each column depicts the images for a different grating. First examine the snapshots in the square-wave column. Note that from time 1, to time 2, to time 3, to time 4, the image ought to appear to drift leftward. Inspect the animation at the bottom of the square column, and you'll see the expected leftward motion.

Now examine the snapshots in the sine-wave column. Again, the image ought to appear to drift leftward. Inspect the animation at the bottom of the sine column, and you'll again see the expected leftward motion.

Now examine the snapshots in the missing-fundamental column. Again, the image ought to appear to drift leftward. But inspect the animation at the bottom of the missing fundamental column, and this time you'll probably see motion rightward. This is the missing-fundamental illusion (Adelson, 1982): the perceived direction of motion is opposite to the direction in which the image features actually move. (You might occasionally perceive veridical motion in the missing-fundamental column, but in the laboratory, the prominent motion direction is the illusory one.)

Reference
Adelson, E. H. (1982). Some new illusions and some old ones analyzed in terms of their Fourier components. Investigative Ophthalmology and Visual Science (Suppl.), 22, 144.

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