Pupil Dynamics and Causality Perception: Insights From Pupillometry

Causality perception is fundamental to interpreting interactions between objects in the physical world. However, little is known about whether physiological responses, particularly pupil size, can implicitly track causality perception. This study employed pupillometry across three experiments to investigate the relationship between pupil dynamics and causality perception. The results revealed that spatiotemporally contiguous launching events (i.e., direct launching), perceived as a causal collision between two objects, induced greater pupil dilation after the collision than simple single-object motion (i.e., passing and pass-by events) or motion with a temporally inverted cause-effect order (i.e., temporal-inverted events), both of which lacked a causal structure. However, launching with a spatial gap (i.e., gap launching) also elicited pupil dilation comparable to direct launching, although gap launching was rated lower in perceived causality. Temporal-inverted events provoked early pupil dilation, corresponding to the sudden and spontaneous motion of the first object. Furthermore, for invariant visual stimuli that could be perceived as either causal launching or noncausal passing (i.e., ambiguous events), pupil size changes did not differentiate between subjective causal and noncausal judgments. These findings indicate that although pupil dilation was evident during causality perception, it was not uniquely or directly tied to causality perception but was influenced by multiple factors, particularly responses to spontaneous motion. This study deepens the understanding of the physiological mechanisms underlying causality perception while also emphasizing the limitation of using pupillometry to examine it.