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Min Bao, Stephen Engel; Separate mechanisms for long- and short-term contrast adaptation. Journal of Vision 2011;11(11):1011. doi: 10.1167/11.11.1011.
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Depriving the adult visual system of contrast at an orientation increases sensitivity to that orientation. Such contrast adaptation strengthens as the inducing period lengthens, with long periods producing longer-lasting effects. This could occur because over time, either a single mechanism gains strength, or additional long-term mechanisms become active. We tested between these alternatives by measuring effects of short-term adaptation before and after long-term adaptation. Subjects were deprived of vertical energy for 4 hours, viewing the world through an “altered reality” system comprised of a head mounted video camera, a computer, and a head-mounted display. Vertical energy was filtered from the video, allowing subjects to interact with the world while being deprived of an orientation. Following this long-term adaptation, which prior work has shown generates large adaptation effects, subjects “deadapted” over the short-term by viewing unfiltered images for 15 min. Subjects then “readapted”, viewing filtered images for 8 min. We measured effects of adaptation using the tilt aftereffect. An initial control period of adaptation, lasting 8 min, showed a weak tilt aftereffect of ~2 deg, lasting 100 sec. Following the four hours of adaptation and 15 min of deadaptation, subjects showed no tilt aftereffect. However, the subsequent 8 min adaptation period generated a large, 3 deg aftereffect, that lasted over 5 min. Together, these results are inconsistent with a single mechanism controlling adaptation, because identical short-term adaptation periods generated different effects depending upon long-term adaptation history. The simplest account of our results is that 4 hours of adaptation produced effects in both short- and long-term mechanisms. Short-term deadaptation then produced a negative effect that masked ongoing long-term adaptation. The 8-min readaptation reestablished the short-term effect and unmasked the long-term effect. Thus, deadaptation and readaptation affected short-term adaptation independent of long-term adaptation, demonstrating that different time-scales are controlled by separate mechanisms.
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