Mach bands

Definition: Optical illusion seen at junction of a dark and a light tone in which there appears to be a light area or band in the dark tone and a dark area or band in the light tone. * The junction between the dark and the light may be abrupt or, as originally observed, gradual. * Also known as Mach rings.
Clearer limits
Mach bands are the psycho-physiological sign of a neuronal mechanism which tries to improve edge definition i.e. to help one see more clearly, and it's one which digital techniques such as unsharp masking imitate. The physiological mechanism is called lateral inhibition: a bunch of excited neurons in the eye tends to inhibit the sensitivity of neighbouring neurons.

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