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Pupil size reveals the perceptual quality and effortless nature of synesthesia.

Christoph Strauch, Casper Leenaars, Romke Rouw

Elife August 4, 2026 DOI: 10.7554/elife.110390 (opens in new tab)

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AI-extracted from the abstract
Characteristics Observational case-control study Peer reviewed
Sample size 16
Population Grapheme-color synesthetes and two matched control groups
Topics Philosophy of mind
Keywords Consciousness Cross-modal None Perception Pupil size Synesthesia
Key findings Pupil responses tracked the brightness of synesthetic colors under constant physical visual input and scaled with self-reported strength, with dynamics comparable to real colors and too rapid to reflect imagery. Controls required to generate color associations showed greater effort-linked pupil dilation than synesthetes or controls who did not report colors, which the authors interpret as evidence that synesthesia is effortless.

Abstract

Synesthesia describes cross-over processes that can generate 'extra' conscious percepts, such as seeing additional color when reading numbers. 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.

Comparable studies

Other observational and cohort studies on philosophy of mind, most cited first.

Study Year Design Participants
The phenomenology of near-death experiences Subjects who reported near-death experiences 1980 Retrospective study n = 78
Visual Phenomenology of the LSD Flashback Persons with a history of LSD use 1983 Observational cohort n = 123
The neurophenomenology of neutral hypnosis. Individuals of high, medium, and low hypnotizability 2013 Observational cohort n = 37
The Phenomenology of Acute Organic Psychosis Patients with acute organic psychosis and acute schizophrenics 1987 Observational cohort n = 148
EEG oscillatory states as neuro-phenomenology of consciousness as revealed from patients in vegetative and minimally conscious states. Brain-damaged patients with disorders of consciousness and healthy-conscious subjects 2012 Observational cohort

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