Separation of edge detection and brightness perception.

When a low spatial frequency noise mask is superimposed onto a luminance staircase, the perceived brightness pattern is dramatically altered although the edges remain visible. We measured contrast thresholds for the edges and for the illusory scalloping (Chevreul-illusion), as a function of noise center spatial frequency. The masking tuning functions overlapped, but peaked at different spatial frequencies and contrast levels. The results suggest that perceived brightness is triggered only by the low spatial frequency components of the edges--the high spatial frequency components are not able to produce a brightness pattern.