UNLABELLED: 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. CONCLUSION: Mach bands are not seen at the edges of an abrupt step change in luminance because the sharp edges actively suppress them.
UNLABELLED: 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. CONCLUSION: Mach bands are not seen at the edges of an abrupt step change in luminance because the sharp edges actively suppress them.