Picture perception: effects of luminance on available information and information-extraction rate

In each of four experiments, complex visual stimuli--pictures and digit arrays--were remembered better when shown at high luminance than when shown at low luminance. Why does this occur? Two possibilities were considered: first that lowering luminance reduces the amount of available information in the stimulus, and second that lowering luminance reduces the rate at which the information is extracted from the stimulus. Evidence was found for both possibilities. When stimuli were presented at durations short enough to permit only a single eye fixation, luminance affected only the rate at which information is extracted: decreasing luminance by a factor of 100 caused information to be extracted more slowly by a factor that ranged, over experiments, from 1.4 to 2.0. When pictures were presented at durations long enough to permit multiple fixations, however, luminance affected the total amount of extractable information. In a fifth experiment, converging evidence was sought for the proposition that within the first eye fixation on a picture, luminance affects the rate of information extraction. If this proposition is correct and, in addition, the first eye fixation lasts until some criterion amount of information is extracted, then fixation duration should increase with decreasing luminance. This prediction was confirmed.