Suprathreshold contrast perception across differences in mean luminance: effects of stimulus size, dichoptic presentation, and length of adaptation.

Contrast constancy across changes in mean luminance was reported to hold over a wide range of luminances in a few studies and to be limited to approximately 1 log unit in another. The studies reporting contrast constancy over a wide luminance range used extended grafting stimuli presented dichoptically (bright stimulus to one eye and dim stimulus to the other) with long adaptation periods. The study reporting only limited constancy used narrow (1-octave-wide) Gabor patches presented side by side to both eyes with only a short (up to 5 s) period of adaptation. The current study was designed to determine whether differences in stimulus bandwidth, presentation format, or adaptation time could account for the different results reported. It was found that increasing stimulus size had no effect on the results. Dichoptic presentation with either a filter in front of one eye or calibrated screen luminance could account for the differences between the studies. When dichoptic presentation was combined with short adaptation periods (of a few seconds) an intermediate deviation from constancy was demonstrated. This effect suggests that the deviations from constancy demonstrated under free viewing are due to a lack of fast local adaptation and not to long-distance interactions across the retina.