The prevailing explanation of Mach bands, the paradoxical bands of light and dark seen where luminance gradients meet plateaux, is that they are due to lateral inhibition in the visual system1-4. This explanation equates Mach bands with distortions in a processed luminance distribution due to selective attenuation of low frequency components. But square waveforms exhibit no Mach bands2,5,6, although they should also be distorted after processing. Measurements of the contrast required to see Mach bands in trapezoidal waveforms and manipulations of their spectra lead us to conclude that phase relationships between Fourier components are important to the structure we perceive. A model based on the odd and even symmetry of visual receptive fields explains our results.
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