Modulation of glare illusion perception and pupillary responses with visual clarity and stimulus size

We investigated the robustness of the glare illusion across variations in visual clarity and stimulus size using pupillary responses as an objective physiological measure. In Experiment 1, visual acuity was manipulated through vision correction, and five stimulus sizes were presented to examine how vision condition influences relative brightness judgments and pupillary responses. In Experiment 2, visual clarity was manipulated using Gaussian blur, and three stimulus sizes were tested under stricter fixation control. Fourteen participants viewed controlled visual stimuli while their pupil diameter was recorded. In Experiment 1, uncorrected vision decreased the probability of judging the glare stimulus as brighter than the control stimulus, suggesting that reduced visual acuity made the glare and control configurations less distinct. However, this effect on baseline-corrected pupillary responses was limited. In Experiment 2, increasing blur decreased the probability of judging the glare stimulus as brighter, indicating that stimulus-level degradation also reduced the perceptual distinction between glare and control stimuli. In contrast, glare stimuli continued to elicit greater pupil constriction than control stimuli did under blurred conditions. Larger stimuli also increased the difference in pupil constriction between glare and control stimuli, suggesting that stimulus size strengthens glare-related pupillary responses. Together, these findings indicate that the glare illusion remains observable as visual clarity changes, although reduced visual clarity can weaken relative brightness judgments. They also show that stimulus size can improve glare-related pupillary responses. These results clarify the boundary conditions for reliably eliciting the glare illusion and help explain interactions between visual clarity and brightness-related pupillary responses.