Visual appearance and sensitivity are mediated by distinct mechanisms

Identifying a visual stimulus and sensitivity to changes in its features have different requirements. Thus, it is possible that different mechanisms underlie appearance and sensitivity judgments of visual stimuli. Here, we tested this possibility using a complex scene where two patches with physically identical luminances appeared to have different lightness. Human participants first judged the perceived contrast of incremental and decremental gratings superimposed on the patches. Next, we measured detection thresholds. Finally, fMRI activity was recorded in response to gratings on these patches. We found that incremental, but not the decremental gratings, appeared to have higher contrast when superimposed on the perceptually lighter patch compared to the darker. However, the thresholds were lower for both types of gratings superimposed on the lighter patch compared to the darker. Finally, using fMRI, we found that the activity in the primary visual cortex (V1) aligns well with the results of the detection task. These results suggest that partly distinct mechanisms underlie sensitivity and appearance and, further, that V1 plays an important role in sensitivity judgments.