Investigating smiling asymmetries in Parkinson's disease through the whistle-smile reflex

Hypomimia is a hallmark of Parkinson's disease (PD), but whether emotional expressions show lateralized abnormalities remains unclear. While asymmetry has been described for posed smiles, no study has examined true spontaneous smile asymmetry in PD. The whistle-smile reflex (WSR), an involuntary smile elicited after voluntary whistling, offers an opportunity to investigate a form of smiling that closely resembles typical spontaneous expressions of joy. In the present study, we aimed to assess the frequency, intensity, and asymmetry of smiling elicited by the WSR in PD compared with healthy controls (HCs) and to explore links between facial and motor asymmetry. Fifty PD patients and 22 age-matched HCs underwent standardized video recording of the WSR. Only participants judged as smiling by ≥ 3/4 raters were included in the chimeric-face analysis. Left-left (LL) and right-right (RR) chimeric images were generated, and seven blinded raters evaluated smile presence and hemiface expressivity. Facial Laterality Index (FLI) and Body Laterality Index (BLI) quantified asymmetries. WSR was less frequent in PD than HCs (video: 44% vs. 72%; mouth-only chimeras: 50% vs. 81%, p < 0.01). PD smiles were more often symmetrical (59% vs. 25%, p = 0.03), with no consistent left-right dominance. Facial and motor asymmetry were unrelated (p > 0.05). Symmetric smilers had higher UPDRS-III scores (p = 0.04). In PD, WSR is reduced and bilaterally flattened, with no lateralized asymmetry, possibly reflecting limbic-subcortical dysfunction. Smile symmetry increases with motor severity, supporting its potential as a simple marker of hypomimia. The WSR and chimeric-smile analysis provide practical tools to assess emotional expressivity in PD.