Contrast discrimination under temporally varying contrast conditions

Psychophysical contrast discrimination of a 0.8-cpd vertical grating was te sted using a paradigm that alternated test and masking gratings at 8 Hz. Ma sking contrasts were lower than, equal to, or higher than the test contrast s. Six test contrasts were combined factorially with six masking contrasts to generate a series of six contrast increment threshold versus test contra st curves (tvc curves). A particularly simple relationship existed between these curves. The curves could be brought into alignment by shifting them d iagonally by the ratio of their masking contrasts. It is shown that this be havior is predicted by a model in which contrast gain is set by the average of the test and masking contrasts coupled with a simple model of contrast discrimination. Contrast gain control integrates contrast over a period of at least 125 msec, and contrast discrimination is a function of this time-a veraged contrast.