It took me twenty years to understand that not everyone saw numbers in color. To me, 3 has always been brick red, 7 a deep midnight blue, and Monday a pale shade of ochre. It was as obvious as saying that the sky is blue. Only when I happened to read a neurology article did I realize: I was a synesthete, and my way of perceiving the world was not universal.

Synesthesia — from the Greek syn (together) and aesthesis (sensation) — is a neurological phenomenon in which stimulation of one sense automatically triggers an experience in another. You hear a musical chord and see a color. You read a word and taste something. You touch a texture and perceive a sound. It is not a metaphor, a hallucination, or poetry. It is literally what the brain does, involuntarily and consistently.

Not as rare as people think

Around 4% of the world's population may have synesthesia, or one person in twenty-five. It is far from a marginal phenomenon. Yet the vast majority of synesthetes do not know it — or at least have never named it. They simply assume everyone works the way they do, until an ordinary conversation one day reveals the difference.

About fifty forms of synesthesia have been documented. The most common is grapheme-color synesthesia: letters and numbers are perceived as colored. Next comes chromesthesia, in which sounds — especially music — trigger colored visions or geometric shapes. Other forms are rarer: some people taste the words they read, while others see colors linked to days of the week, months, or even the personalities of those around them.

Geniuses who saw differently

Art history is filled with synesthetic creators. Wassily Kandinsky is the best-documented example. In 1896, while attending a performance of Wagner's Lohengrin in Moscow, he literally saw shapes and colors emerge from the music. The experience changed his life and his art: his abstract paintings are not purely visual compositions — they are transcriptions of sensations. Kandinsky called his works “compositions,” directly referring to the music he heard while painting.

Arthur Rimbaud wrote in his 1871 poem Vowels: “A black, E white, I red, U green, O blue: vowels”. Although specialists still debate whether he was a synesthete, the precision and consistency of his color associations strongly suggest a lived experience rather than a simple poetic metaphor.

Contemporary music also offers many accounts: Stevie Wonder, Duke Ellington, Pharrell Williams, Lady Gaga, and Billie Eilish have all publicly described their synesthesia. Pharrell Williams, in particular, perceives every musical note as a specific color — a trait he says has been decisive in his music production choices.

What happens in the brain

For a long time, the scientific community treated synesthesia with skepticism. Was it a genuine perception or merely an imaginary association cultivated since childhood? Modern neuroscience has settled the question: it is real.

Researchers have identified two main mechanisms. The first is cross-activation between neighboring sensory areas in the cortex. In grapheme-color synesthetes, the regions that process visual shapes and those that process color are unusually connected — an additional link that simultaneously activates color perception when a number or letter appears.

The second mechanism is disinhibition: the human brain naturally contains multisensory connections that most people unconsciously filter and suppress. In a synesthete, this filter may be less active, allowing through associations that other people block without realizing it.

Genetics also plays a role: synesthesia is hereditary and tends to occur within the same families, although its exact form may vary from one family member to another.

An experience you do not choose

What distinguishes true synesthesia from a simple imaginary association is that it is involuntary, automatic, and stable over time. Camille's 3 will always be red, and that red will not change from one year to the next. This is one of the criteria researchers use to validate the phenomenon: they test the consistency of associations over many years.

Synesthesia is not a disorder — it does not disrupt daily life and is not medically treated. It is simply a variation in perception. Most synesthetes experience it as an enrichment, and sometimes even an advantage: some remember phone numbers more easily because they “see” them in color, while others memorize people's names through the shade they associate with them.

Another person's sensory world

What fascinates me about synesthesia — and I imagine anyone who discovers it feels the same — is what it reveals about the nature of perception in general. We all think we perceive the world in the same way. We use the same words for the same colors and sounds. Yet what we actually experience inside when we perceive something can differ considerably from one person to another.

Synesthesia is only the most visible and best-documented form of this perceptual diversity. It raises, in a very concrete way, an old philosophical question: how can I know that what you see as “red” resembles what I see as “red”? We have simply learned to agree on the words — not necessarily on the experiences they describe.

Perhaps synesthesia is not so much an anomaly as a window onto something we all do, to varying degrees, without being aware of it.