Mach bands are an optical illusion consisting of an image of two wide bands, one light and one dark, separated by a narrow strip with a light-to-dark gradient. The human eye perceives two narrow bands of different brightnesses either side of the gradient that are not present in the original image. The illusion is named after Ernst Mach.
The effect is like that of a spatial high-boost filter. It is usually supposed that it is caused by lateral inhibition of the receptors in the eye. An alternative explanation is that visual perception, using a fundamentally statistical strategy, represents the common occurrence of highlights and lowlights in association with luminance gradients.
References
Lotto RB, Williams SM, Purves D (1999). "Mach bands as empirically derived associations". Proceedings of the National Academy of Sciences 96 (9): 5245-50. PMID 10220451.
