Brightness variations in stimuli with ramp-like contours.

Perceived brightness was measured at several points on stimuli bounded by luminance ramps. As the ramps widen, the perceived contrast between stimulus and background decreases, although objective contrast is unaffected. The rate of decrease varies from one part of the stimulus to another, and complex variations in perceived brightness, including Mach bands, occur. A von Bekesy-type model accounts for many features of the data.