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While existing research focuses on the mechanisms and effects of synesthetic associations, it often overlooks its most distinctive feature: unique sensory phenomenology. Here, we introduce pupillometry as an objective physiological measure of synesthetic color phenomenology. Across 16 grapheme-color synesthetes and two matched control groups, pupil responses tracked the brightness of synesthetic colors under constant physical visual input, scaling with self-reported strength. Synesthetic colors elicited pupil dynamics comparable to real colors, dissociating synesthetes from non-synesthetes. These responses emerged too rapidly to reflect imagery and scaled with reported color brightness, revealing cross-over caused genuine perceptual processing. Controls required to generate color associations showed greater effort-linked pupil dilation than synesthetes or controls who did not report colors, providing evidence for the effortless nature of synesthesia. Synesthesia thus provides a tractable human model for studying physiologically measurable phenomenology.<\/jats:p>","DOI":"10.7554\/elife.110390","type":"journal-article","created":{"date-parts":[[2026,3,6]],"date-time":"2026-03-06T18:30:15Z","timestamp":1772821815000},"update-policy":"https:\/\/doi.org\/10.7554\/elife.110390","source":"Crossref","is-referenced-by-count":0,"title":["Pupil size reveals the perceptual quality and effortless nature of synesthesia"],"prefix":"10.7554","volume":"15","author":[{"ORCID":"https:\/\/orcid.org\/0000-0002-6380-8635","authenticated-orcid":true,"given":"Christoph","family":"Strauch","sequence":"first","affiliation":[{"id":[{"id":"https:\/\/ror.org\/04pp8hn57","id-type":"ROR","asserted-by":"publisher"}],"name":"Experimental Psychology, Helmholtz Institute, Utrecht University","place":["Utrecht, 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