Abstract: A new phenomenon of odd harmonic squarewave gratings was obser- ved when the stimulus was a rapidly travelling squarewave grating. Some stationary harmonic gratings could be observed at some critical temporal frequencies of the test grating, however at frequencies slightly different from the critical ones, the harmonic gratings appeared to be moving. Th- eir apparent motion directions were the same as that of the test grating or opposite to it, depending on whether the temporal frequencies of the test grating were greater or lower than the critical frequencies. Some sug- gestions concerning the spatio-temporal sampling theory and Fourier ana- lysis were proposed in this paper.

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