Abstract: PURPOSE: Causality is fundamental to understanding our physical environment. Previous research typically measured causal perception by various behavioral indicators, but little is known about whether causal perception can be implicitly detected by physiological responses. This study aimed to investigate the impact of causal perception on pupil responses under explicit or ambiguous conditions.
METHODS: Participants viewed animations featuring two discs (A and B) moving sequentially and judged whether the event represented a launch (A caused B to move) or a pass (A passed the stationary B). Experiment 1 manipulated the overlap between A and B across nine conditions (0% to 100%, in steps of 12.5%). Experiment 2 included three overlap conditions (0%, 100%, and ambiguous), with the ambiguous condition determined by the point of subjective equality (PSE) obtained in Experiment 1. Pupil size was recorded during Experiment 2.
RESULTS: Experiment 1 revealed participants tended to perceive the animations as launching, as evidenced by significantly larger PSEs than 50%. In Experiment 2, when the overlap led to explicit causal perception, participants’ pupils dilated after the second disc completely overlapped the first disc and the first disc appeared to pass through it. Under the ambiguous condition, participants’ pupils dilated after the animations ended and prior to launching reports. Notably, these pupil effects were not driven by the baseline pupil size, suggesting they were induced by cognitive processing rather than general arousal.
CONCLUSIONS: The current study revealed distinct pupil responses during causal perception based on unambiguous versus ambiguous sensory evidence, implying the involvement of different cognitive processes in these two situations.

Key words: causal perception, launching, passing, pupil size