Synesthesia: From Ancient Greece to Modern Neuroscience, a Journey Through Sensory Perception

UC Berkeley Professor Liesl Yamaguchi's book, *On the Colors of Vowels: Thinking Through Synesthesia*, explores the history of synesthesia, a neurological condition where stimulation of one sense triggers an experience in another. The first documented mention appeared in 1812 in a Bavarian medical student's dissertation, detailing associations between musical tones, letters, and colors; for example, "A and E: vermilion, I: white, O: orange and so forth." While the term "synesthesia" was used in ancient Greece to describe a simultaneous feeling between two people, its modern usage dates to the late 19th century. Yamaguchi investigates how the concept emerged, despite evidence suggesting it's an "age-old phenomenon." The most recognized forms of synesthesia involve seeing colors with musical sounds or linguistic elements. Yamaguchi focused on vowels, intrigued by poets describing poetic sounds in visual terms, such as "coloration of a rhyme" or "dark vowel." Georg Sachs' 1812 dissertation is considered the first modern report on synesthesia. Before this, there is no documentation of this way of sensing. Yamaguchi suggests examining historical texts to find "glimmers" of what is now called "synesthesia." In the 19th century, visual descriptions of vowels appeared across various fields, including experimental psychology, acoustics, and linguistics. However, perceiving non-verifiable sensations was stigmatized, often classified as "mental disturbances." Records were primarily found in diaries, letters, or anonymous accounts. The 20th century transformed synesthesia into a scientific object, requiring rigorous definition and testability. It shifted from a mental disturbance to a sign of genius, associated with exceptional creativity. However, no one in the 19th century identified as a synesthete because the concept hadn't been codified yet. Until the 21st century, proving synesthesia was challenging due to its subjective nature. Brain scans showing unusual activity in sensory cortices provided external validation. The scientific codification of synesthesia in the 20th century led to a loss of variability in the phenomenon. Advances like CT scans and fMRI demonstrated aberrant activity in the visual cortex in response to sonorous stimuli, offering scientific proof. This allowed for more flexible study, bringing scientific and humanistic discourses closer. Yamaguchi defines synesthesia as a "cluster concept" about complex sensory interactions. She emphasizes the importance of language in understanding how we sense things, viewing language as an archive for those who know how to read it.