Altered causality perception in Parkinson’s disease

Causality perception allows observers to infer causal relationships from dynamic visual events. A classic example is Michotte’s launching effect, in which one moving object contacts a stationary object and stops as the second object begins to move, creating the perceptual impression that the first object has launched the second. Here, we investigated whether this form of visual causality perception is altered in Parkinson’s disease (PD), a neurodegenerative disorder associated with motor and cognitive impairments. Using a psychophysical paradigm, we compared causality judgments in PD patients and age-matched healthy controls while systematically varying the spatial overlap between the two objects. Participants judged whether the event appeared as a causal “launch” or a non-causal “pass-through.” More overlap usually reduces the sensation of a “launch” event in favor of a “pass-through” in this task. Compared with controls, PD patients showed a significant shift in the point of subjective equality toward lower overlap values, indicating a group-level bias away from causal interpretations. They also showed shallower psychometric slopes, suggesting reduced sensitivity to overlap cues that normally support causal judgments. Two control experiments ruled out alternative explanations based on low-level perceptual differences: spatial overlap perception was intact in PD, and causality judgments in healthy controls remained stable across multiple stimulus speeds. Together, these findings suggest that PD is associated with altered visual causality perception, reflecting disrupted integration of sensory evidence into causal interpretations rather than a basic deficit in overlap or speed perception.