The "pincushion grid" illusion.
The illusion generated by a "pincushion grid" is not predicted from the two-dimensional Fourier transform of the grid. This implies that the visual system may not perform two-dimensional Fourier transforms of observed patterns.
D005583:Fourier Analysis; D006801:Humans; D007088:Illusions / Q000502:physiology*; D009903:Optical Illusions / Q000502:physiology*