Abstract: PURPOSE: The circadian rhythm regulates visual perception in both human and animals. Although certain studies have suggested training subjects at roughly the same time in their experimental protocols, likely a reflection of researchers' concern about the potential interference of circadian rhythm, a systematic investigation into the impact of circadian rhythm on perceptual learning remains lacking.
METHODS: In the current study, we adopted two classical visual tasks, e.g. contrast detection and Vernier offset discrimination, to investigate whether circadian rhythm affect visual perceptual learning. Fourty participants, all of intermediate chronotype, were divided into two groups and underwent six training sessions that evenly allocated in two different temporal orders: three morning then switched to three evening sessions, and three evening to three morning sessions.
RESULTS: A multi-component perceptual learning model analysis (Yang et al., 2020, 2022) revealed that, for contrast detection task, individuals in the evening-morning temporal order group exhibited lower initial thresholds than those in the morning-evening group did; On the other hand, for Vernier offset discrimination task, individuals in the morning-evening group had significantly lower initial thresholds than those in the evening-morning group did. Nevertheless, no significant between-group differences in learning speed and other components in the accumation of learning effects (e.g. overnight gain and relearning) were detected in either of the tasks.
CONCLUSIONS: Our findings suggested that the circadian rhythm only influence subject’s initial performance at the beginning of perceptual learning, and the subsequent learning process was likely immune to circadian rhythm, signifying a dissociation between circadian rhythm and visual perceptual learning. Moreover, the influence of the circadian rhythm on initial performance is probably task-specific.

Key words: Visual perceptual learning, Plasticity, Circadian rhythm