Separate mechanisms for long- and short-term contrast adaptation

Abstract

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.