Visual illusions without low spatial frequencies.

It has been suggested that many geometrical illusions may be caused by the lower spatial frequencies in the Fourier spectra of these images. We have tested this hypothesis by constructing classical illusory figures out of a new dot stimulus, which is free of visible low spatial frequencies. In every case, not only does the illusion persist, but for the Müller-Lyer figures, which were measured quantitatively, the illusion magnitude is not significantly changed. We conclude that geometrical illusions are not primarily a consequence of low spatial frequencies in the illusory figures.