PURPOSE: Visual environment changes at multiple timescales. It is thus hypothesized that adaptation mechanisms operate at differing timescales to accommodate the environmental changes. This has been demonstrated in recent work. However, neural substrates of these adaptation mechanisms remain unclear. To address this issue, two psychophysical experiments were conducted.
METHODS: Using the interocular suppression paradigm, in Experiment 1a and 1b we firstly investigated the timescales of adaptation before and after cortical processing stage by tracking the time courses of contrast adaptation effects under different viewing conditions(monocular, binocular, and dichoptic). To compare the timescales of mechanisms spanning wider in the visual processing hierarchy, in Experiment 2 we further tracked the decay of tilt aftereffect after adaptation to either a compound grating pattern (curvature) or its component gratings.
RESULTS: Results of Experiment 1a demonstrated that slower adaptation mechanisms were more predominant for the monocular condition than for the binocular condition. Results of Experiment 1b also showed that the residual adaptation effects were larger in the monocularly adapting condition and the dichoptically orthogonal adapting condition than that in the binocular adapting condition. Considering that interocular suppression is stronger in the binocular condition, and it mostly occurs on the cortical level, results of Experiment 1a and 1b might indicate that adaptation mechanisms at the binocular processing stages were longer-term. Further, the results Experiment 2 revealed a slower decay for the compound [mid-level (e.g. V4)] than for the component (early visual areas) adaptation condition, suggesting longer-term mechanisms in the mid-level (e.g. V4) than the early visual areas.
CONCLUSIONS: Our findings indicate that neural mechanisms controlling visual adaptation may become more sluggish along the visual processing stream, at least from low- to mid-level processing stages.