Distinct mechanisms control contrast adaptation over different timescales.

Abstract

Exposure to a high contrast pattern reduces visual sensitivity to similar patterns. Such contrast adaptation becomes longer-lasting as the inducing period lengthens. This might occur because, over time, either a single mechanism controlling adaptation gains strength, or additional mechanisms operating over longer timescales become active. We resolved this debate using a "deadaptation" procedure to reveal separate mechanisms acting at distinct timescales. Subjects adjusted an adapted sinusoidal grating, presented in either the upper or lower visual field, to match the appearance of a 0.25 contrast unadapted grating, presented in the opposite visual field. The patterns were presented for 200 msec, following a 1400 msec adapting grating presentation. The experiment had four stages: First, a 0.25 contrast adaptor was presented during 2 minutes of baseline measurements. This was followed by 5 minutes presentation of high contrast (0.80) adaptation, then 40 seconds of low contrast (0.0625) adaptation, and a final 2 minutes of 0.25 contrast adaptation. The 5 minutes of high contrast adaptation showed strong effects. Subjects increased the contrast of the adapted pattern substantially to match the unadapted pattern (by 0.17 above baseline measurements). These effects were completely eliminated by the brief low contrast adaptation (deadaptation). However, in the final 0.25 contrast adaptation period, effects of the high contrast adaptation gradually reappeared (asymptoting at 0.045 above baseline, p <0.01). A single mechanism cannot account for such spontaneous recovery of adaptation. The simplest explanation is that deadaptation produced a negative effect in shorter-term mechanisms that masked ongoing adaptation in longer-term mechanisms. Once the adapting contrast returned to baseline, the short-term negative effect decreased quickly, allowing the ongoing longer-term effect to be unmasked. We found similar results using contrast detection and tilt-aftereffect paradigms. Distinct mechanisms operating over different timescales may give vision needed flexibility in adjusting to constantly changing environments. Meeting abstract presented at VSS 2012