Two contrast adaptation processes: contrast normalization and shifting, rectifying contrast comparison.

We present psychophysical results demonstrating the interaction of two contrast adaptation processes in human vision: (1) A contrast-gain-control process of the normalization type and (2) a recently-discovered shifting, rectifying contrast-comparison process. Observers adapted (for 1 s) to a grid of Gabor patches at one contrast, then a brief (94 ms) test pattern was shown, and then the adapt pattern was shown again (1 s). The test pattern was the same as the adapt pattern except that the Gabor patches had two different contrasts arranged to create vertical or horizontal contrast-defined stripes. Observers identified the orientation of the test pattern's stripes. Performance is a complicated ("butterfly shaped") function of the average test contrast, centered at the adapt contrast. This shape is a consequence of the interaction of the two contrast adaptation processes. At the ends of the function are "Weber zones" in which the contrast-gain-control process dominates, and at the center of the function is a "Buffy zone" in which the recently-discovered contrast-comparison process dominates.