Why Mach bands are not seen at the edges of a step.
Mach bands are seen at the two ends of a ramp in luminance from one uniform level to another. Narrow, sharp-edged stimuli centered on the ramp attenuate both Mach bands simultaneously.


CONCLUSIONS
Mach bands are not seen at the edges of an abrupt step change in luminance because the sharp edges actively suppress them.
D005556:Form Perception / Q000502:physiology*; D006801:Humans; D007088:Illusions / Q000502:physiology*; D009433:Neural Inhibition; D009903:Optical Illusions / Q000502:physiology*; D010364:Pattern Recognition, Visual / Q000502:physiology*; D011601:Psychophysics