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Stephen Engel; Mechanisms of multi-hour and multi-day contrast adaptation. Journal of Vision 2014;14(15):9. doi: 10.1167/14.15.9.
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Vision adapts powerfully to many aspects of its environment, including contrast. But can adaptation produce truly large and long-lasting effects? The limits of contrast adaptation remain unexplored, as adaptation durations in most studies last only a few minutes. We adapted subjects to reduced contrast for hours and days. Subjects performed everyday activities while wearing a camera whose video was filtered in real time, and viewed on a head-mounted display. Filtering removed vertical energy from the images. Subjects judged the orientation and contrast of test stimuli, yielding estimates of changes in perceived tilt and contrast due to adaptation. Our results revealed distinct mechanisms of adaptation operating at a wide range of time-scales. In one experiment, 4 hours adapting to lack of vertical was followed by brief adaptation to unaltered images. The brief adaptation canceled effects of the 4 hours, but continued testing in a neutral environment revealed their spontaneous recovery. This pattern indicates that effects of 4-hour adaptation were controlled by medium-term mechanisms distinct from those controlling the short-term adaptation to normal images. In a second series of experiments, subjects adapted for 4 days. Toward the end of the first day, adaptation showed a drop in strength, which was surprising because the adapting environment remained present. This result suggests that classical short- and medium-term effects of contrast adaptation may weaken after approximately one day. Effects on days two through four showed further strengthening of adaptation, indicating that a distinct slow adaptive mechanism can overcome the factors limiting shorter-term adaptation.
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