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While asymmetry has been described for posed smiles, no study has examined true spontaneous smile asymmetry in PD. The whistle\u2013smile 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 \u2265\u20093\/4 raters were included in the chimeric-face analysis. Left\u2013left (LL) and right\u2013right (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%,\n                      <jats:italic>p<\/jats:italic>\n                      \u2009&lt;\u20090.01). PD smiles were more often symmetrical (59% vs. 25%,\n                      <jats:italic>p<\/jats:italic>\n                      \u2009=\u20090.03), with no consistent left\u2013right dominance. Facial and motor asymmetry were unrelated (\n                      <jats:italic>p<\/jats:italic>\n                      \u2009&gt;\u20090.05). Symmetric smilers had higher UPDRS-III scores (\n                      <jats:italic>p<\/jats:italic>\n                      \u2009=\u20090.04). In PD, WSR is reduced and bilaterally flattened, with no lateralized asymmetry, possibly reflecting limbic\u2013subcortical 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.\n                    <\/jats:p>\n                  <\/jats:sec>","DOI":"10.1007\/s00702-026-03193-1","type":"journal-article","created":{"date-parts":[[2026,7,2]],"date-time":"2026-07-02T17:28:39Z","timestamp":1783013319000},"page":"2535-2546","update-policy":"https:\/\/doi.org\/10.1007\/springer_crossmark_policy","source":"Crossref","is-referenced-by-count":0,"title":["Investigating smiling asymmetries in Parkinson\u2019s disease through the whistle\u2013smile reflex"],"prefix":"10.1007","volume":"133","author":[{"given":"Adriana","family":"Martini","sequence":"first","affiliation":[],"role":[{"vocabulary":"crossref","role":"author"}]},{"given":"Giulia","family":"Paparella","sequence":"additional","affiliation":[],"role":[{"vocabulary":"crossref","role":"author"}]},{"given":"Simone","family":"Aloisio","sequence":"additional","affiliation":[],"role":[{"vocabulary":"crossref","role":"author"}]},{"given":"Luca","family":"Angelini","sequence":"additional","affiliation":[],"role":[{"vocabulary":"crossref","role":"author"}]},{"given":"Anna Sofia","family":"Grandolfo","sequence":"additional","affiliation":[],"role":[{"vocabulary":"crossref","role":"author"}]},{"given":"Martina","family":"De Riggi","sequence":"additional","affiliation":[],"role":[{"vocabulary":"crossref","role":"author"}]},{"given":"Daniele","family":"Birreci","sequence":"additional","affiliation":[],"role":[{"vocabulary":"crossref","role":"author"}]},{"given":"Davide","family":"Costa","sequence":"additional","affiliation":[],"role":[{"vocabulary":"crossref","role":"author"}]},{"given":"Luca","family":"Marsili","sequence":"additional","affiliation":[],"role":[{"vocabulary":"crossref","role":"author"}]},{"given":"Matteo","family":"Bologna","sequence":"additional","affiliation":[],"role":[{"vocabulary":"crossref","role":"author"}]}],"member":"297","published-online":{"date-parts":[[2026,7,2]]},"reference":[{"key":"3193_CR49","doi-asserted-by":"crossref","first-page":"745","DOI":"10.1016\/0028-3932(85)90081-8","volume":"23","author":"GL Ahern","year":"1985","unstructured":"Ahern GL, Schwartz GE (1985) Differential lateralization for positive and negative emotion in the human brain: EEG spectral analysis. 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Dr. Marsili has received a grant (collaborative research agreement) from the International Parkinson and Movement Disorders Society for the MDS-UTRS Validation Program (Role: PI), Non-Profit. All other authors have no relevant financial or non-financial interests to disclose.","order":1,"name":"Ethics","label":"Conflict of interest","group":{"name":"EthicsHeading","label":"Declarations"}},{"value":"This study was performed in line with the principles of the Declaration of Helsinki.","order":2,"name":"Ethics","label":"Ethical approval","group":{"name":"EthicsHeading","label":"Declarations"}}]}}