Adaptation to ever-changing environment is a fundamental design to the organisms. For the visual environment, changes happen in different timescales in where the visual system can adapt by adjusting its state. How does the visual system adapt the visual environment with different timescales? Two theories have been proposed for explaining time course of visual adaptation. One is a single mechanism which controls visual adaptation by a single and adjustable timescale. The other is multiple mechanisms which control visual adaptation by operating in different timescales. Although recent studies have showed that multiple mechanisms controlled visual adaptation by measuring the spontaneous recovery, it is unclear whether explicit memory plays a substantial role in these studies. And, neural substrates of multiple mechanisms remain largely unknown, such as whether different brain areas in the visual pathways correspond to different timescales of visual adaptation.
In Study 1, including Experiment 1 and 2, to examine which mechanism (single or multiple) controls the timecourse of contrast adaptation at the unconscious processing levels, we investigated whether the spontaneous recovery of contrast adaptation would emerge at the levels. To address the issue, we used a modified Continuous Flash Suppression (Experiment 1) and visual crowding (Experiment 2) paradigms to render the adapting stimuli invisible, but still observed the spontaneous recovery phenomenon. These results exclude the possibility that spontaneous recovery found in the previous work was merely the consequence of explicit visual memory. Our findings also demonstrate that contrast adaptation, even at the unconscious processing levels, is controlled by multiple mechanisms.
In Study 2, including Experiment 3 and 4, we examined whether multiple adaptation mechanisms correspond to different stages of visual processing. In Experiment 3, we compared the timescales of adaptation between the stages of early and mid-level visual processing by tracking the decay of the curvature aftereffect after adaptation to either a compound stimulus or a component stimulus. The results revealed a slower decay for the compound adaptation condition than for the component adaptation condition, indicating that neural mechanisms for visual adaptation are more sluggish at the mid-level than those at the early stage of visual processing. In Experiment 4, we investigated the timescales of cortical and subcortical adaptation mechanisms by tracking the time courses of contrast adaptation effects under the monocular, binocular and dichoptic viewing conditions. The results indicated that adaptation effects decayed more slowly in the monocular condition than that in the binocular condition. Considering that interocular suppression mostly occurs on the cortical level, the results suggested that adaptation mechanisms at the binocular processing stages were longer-term. In other words, longer-term adaptation mechanisms resided in the cortical than in the subcortical processing stage. Two experiments of Study 2 together support the view that neural mechanisms controlling visual adaptation may become more sluggish along the visual processing stream.
In summary, the present studies showed that (i) contrast adaptation is controlled by multiple mechanisms at the unconscious processing levels; (ii) neural mechanisms for visual adaptation may become more sluggish along the visual processing pathways.