Why Do Some People See Numbers as Colors?

For most people, the number 7 is a number. Black ink on white paper, or whatever color you wrote it in.

For some people, 7 is green. Always green. It’s been green since they learned to count. It’s not a choice or a metaphor — when they see the numeral 7, the experience of greenness is automatic and involuntary, as natural as seeing the shape itself.

This is synesthesia — a blending of the senses — and it’s more common than most people realize.

What It Is

Synesthesia (from the Greek syn, together, and aisthesis, sensation) is a neurological condition in which stimulation of one sense automatically triggers an experience in another. The most common form — affecting roughly 1-4% of the population — is lexical-color synesthesia, in which letters, numbers, words, and sometimes days of the week have specific, consistent colors.

For synesthetes, these associations are:

Automatic. They don’t think about the color. It appears.

Consistent. A synesthete who sees 7 as green will see 7 as green five years later. Ask them to draw their synesthetic colors and show them the same test again a year later — the results are nearly identical. This reproducibility is part of how researchers verify the phenomenon.

Involuntary. Synesthetes can’t turn it off. They see the colors whether they want to or not.

Idiosyncratic. Two synesthetes will rarely agree on which number is which color. Your 7 is green; someone else’s is yellow. The colors are personal.

The last point is important: synesthesia is not a shared system or a cultural artifact. It’s generated internally, uniquely, by each person’s brain.

The Famous Synesthetes

The phenomenon has a well-documented history among artists.

Vladimir Nabokov, the novelist (Lolita, Pale Fire), described his synesthesia extensively, including in his memoir Speak, Memory. He saw letters in colors — specific shades he could describe in exquisite detail. He noted that his mother was also synesthetic, and that her color associations differed from his, which he found interesting rather than contradictory.

Wassily Kandinsky, the abstract painter, is believed to have had strong sound-to-color synesthesia. His paintings were consciously attempts to represent what he “heard” as colors — the work that emerged from this became foundational to abstract art. He wrote extensively about the relationship between music and color in visual composition.

Billy Joel, the musician, has described certain musical keys as having colors. C major is white, E major is yellow. His compositional process reportedly involves these associations.

The disproportionate representation of synesthetes among artists isn’t coincidental. People whose brains make spontaneous, involuntary connections between stimuli and other sensory domains may be naturally predisposed toward art forms that seek to translate one kind of experience into another.

What’s Happening in the Brain

Brain imaging has settled the question of whether synesthesia is “real” — in the sense of involving actual neural differences — or whether synesthetes are just describing metaphorical associations.

When a non-synesthetic person reads the number 7, their visual cortex activates in regions associated with shape and form processing. The color-processing areas (V4 and V8 in the visual cortex) are not particularly active.

When a lexical-color synesthete reads 7, the color-processing areas also activate. The experience of color is neurologically real — it’s happening in the same region of the brain that processes color from the outside world.

The leading mechanistic explanation, developed by neuroscientist V.S. Ramachandran and Edward Hubbard, is cross-activation: the regions of the brain that process numerical shape and those that process color are anatomically adjacent in the inferior temporal lobe. In synesthetes, there may be an unusually strong set of connections — or unusually weak inhibitory signals between them — such that activation of one area bleeds into the other.

A competing theory, the disinhibited feedback model, suggests that these cross-modal connections exist in everyone but are normally suppressed by top-down inhibitory signals. In synesthetes, the inhibition is weaker, allowing associations that the rest of us have but never consciously experience to surface.

You Probably Have a Weaker Version

The strict definition of synesthesia involves automatic, involuntary, specific sensory experiences. But research suggests that most people have some form of cross-modal association that, under the right circumstances, behaves like a weak version of synesthesia.

The most cited evidence: the bouba-kiki effect.

Show almost anyone two abstract shapes — one rounded and soft, one angular and spiky — and tell them that one is called “bouba” and the other is called “kiki.” Ask them to guess which is which.

About 95% of people across all tested cultures assign “bouba” to the rounded shape and “kiki” to the spiky one.

The shapes don’t sound like anything in the conventional sense. But the auditory experience of saying “kiki” — the sharp, angular movements of tongue and mouth, the high-frequency consonants — maps onto the visual experience of sharp, angular shapes. “Bouba” maps onto soft curves.

This is cross-modal association. Most people have it. Most people don’t perceive it as a separate sensory experience — it stays below the level of conscious sensation. Synesthetes have the same basic wiring, just with stronger or less-inhibited connections.

Synesthesia and Learning

There’s a practical dimension worth noting: synesthetes tend to score better on memory tests for number sequences and random lists.

The mechanism is clear enough: if every number has a unique color, a random number sequence is also, for a synesthete, a color sequence. They have an additional layer of encoding — not just shape, but color, not just sound, but visual texture. Memory is associative, and synesthetes have more associations available.

This advantage is context-dependent. It doesn’t help with all tasks, and for some — where the automatic color association conflicts with the correct answer in a color test, for instance — synesthesia is a disadvantage. But for the specific task of encoding arbitrary sequences, having additional cross-modal associations seems to help.

Why It Exists

The honest answer: no one is certain.

Synesthesia appears to be heritable — it runs in families, and the heritability estimates are significant. But the specific color associations don’t match between family members, which means whatever is inherited is a general tendency toward cross-activation rather than the specific connections.

One hypothesis: the same neural flexibility that produces synesthesia in its strong form is, in weaker versions, associated with creative and divergent thinking — making connections between things that aren’t normally connected. If synesthesia is the extreme end of a continuous spectrum, the genes that produce it might be maintained in the population because the mild form is adaptive, conferring some advantages in pattern recognition and creative problem-solving.

The price of the trait, in its strong form, is seeing letters in colors forever.

The benefit, possibly, is that the same wiring makes everything more connected — senses, ideas, memories, forms.

Not a bad trade.