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Pupil size reveals the perceptual quality and effortless nature of #synesthesia https://elifesciences.org/articles/110390 Physiologically measurable phenomenology: "responses emerged too rapidly …

Pupil size reveals the perceptual quality and effortless nature of synesthesia
Synesthetes' pupils reflect the brightness of colors that they only see internally, making their hidden perceptual experience objectively measurable.