Abstract: PURPOSE: Although a number of studies have demonstrated that visual function is regulated by the circadian rhythm, results varied significantly across studies. The current study examined the effect of circadian rhythm on a variety of visual functions in a group of subjects with consistent chronotype, aiming to tackle the discrepancy in the literature.
METHODS: Visual contrast detection, Vernier offset discrimination, and color perception (using Munsell 100-Hue Test) were adopted. To match experimental setup in previous studies, visual detection and discrimination tasks were tested at both central and peripheral retinal locations. Thirty-four participants with an intermediate chronotype were allocated into two groups and completed two test sessions in different temporal orders: morning to evening, and evening to morning.
RESULTS: Indexed by the difference of threshold change between the two test sessions, we observed a significant decrease in threshold for contrast detection task in both central and peripheral visual fields when individuals took tests in the temporal order from morning to evening, and in contrast, a reverse effect (i.e. threshold increase) in the temporal order from evening to morning. For the Vernier offset discrimination task in peripheral visual fields, the average threshold change exhibited an opposite pattern, with individuals took test order of evening to morning showing a significant reduction. For color perception, there was no significant difference in threshold changes between the two testing sessions.
CONCLUSIONS: Visual detection ability was found to be better during nighttime, while visual discrimination ability was superior in the morning. No diurnal variation was detected in color perception within the scope of this study. Our results indicated that the regulatory influence of circadian rhythm on visual perception is function dependent.

Key words: Visual detection, Visual discrimination, Color perception, Circadian rhythm