Synesthesia: when letters have colors and words have tastes

Most of us look at the letter R and see a letter. Some people look at it and also get a color that belongs to it. Scientists studying this give the example “R is sky blue.” Ask again next week, and R is still sky blue.

This is synesthesia. It sounds like magic, but it is real, it can be measured, and it is a lovely window into how our senses are wired together.

What is synesthesia?

Synesthesia is when stimulating one sense also brings an extra experience that nobody stimulated, often in a different sense. A letter brings a color. A sound brings a color. A word brings a taste. Reviews of the research describe three things that make it special: the extra experience is triggered by something real, it happens automatically, and it stays the same over time.

People have written about synesthesia for about 200 years. Yet psychologist Jamie Ward noted in his 2013 review that real progress in studying it came only in the decade or so before he wrote. One 2018 genetics paper calls it “a rare nonpathological phenomenon,” meaning it is not a sickness, and Ward’s review discusses it as an individual difference in how people perceive and think.

Many flavors of synesthesia

Synesthesia comes in many forms. Here are a few that scientists have studied closely.

• Colored letters and numbers. This is called grapheme-color synesthesia, and it is the most studied type. One research team put its steadiness this way: if a is carmine red, it is always carmine red.
• Colored days. Monday, Tuesday and the rest each have their own color. In a 2006 survey, this was the most common type of all.
• Colored sounds. Voices or music bring colors. In a 1987 case study, hearing words brought colors, and numbers had colors of their own too.
• Tasting words. Jamie Ward and Julia Simner studied a person known in their research as JIW, for whom speech sounds bring tastes that are felt in the mouth. The sounds often echo the name of a food: the sounds inside the word “mince” can bring the taste of mince. Meaning can play a part too: the word “blue” tastes “inky.” Everyday noises, on the other hand, bring no taste at all.
• Mirror touch. Watching someone else being touched switches on brain circuits similar to those for real touch. For people with mirror-touch synesthesia, it can feel like a touch on their own body. A 2007 study found that this type goes together with higher scores on empathy.

Even two people with colored letters may not experience them the same way. Some see the colors out in the world, right on the page. Others see them only in the mind’s eye. Researchers call these two groups projectors and associators.

How common is it?

For a long time the textbook answer was “very rare”: about 0.05% of people, or 1 in 2,000, mostly women.

In 2006, Julia Simner, Jamie Ward and their colleagues checked. Instead of waiting for synesthetes to come forward, they sampled people who had not volunteered themselves, and they used objective tests to confirm each case. Synesthesia turned out to be 88 times more common than the old figure, which works out to about 4.4% of people. Colored letters showed up in about 1% of people. And there was no strong difference between women and men. The team suggested that the older lean toward women probably came from, or was made bigger by, women being more willing to talk about it.

Children have it too. In a 2009 study, Simner’s team tested 615 children aged 6 and 7 at 21 primary schools in the UK. By their estimate, a typical UK primary school has two or three children with colored letters at any time. Following the children for a year, the researchers watched the colors settle: pairings that started out chaotic grew into a fixed, steady system.

How do scientists know it’s real?

It’s a fair question. How can anyone check a color that only one person sees?

Ask again later. In 1987, Simon Baron-Cohen and colleagues confirmed a case of colored hearing by checking that the colors stayed the same across time. They also found that the colors of made-up words came from the colors of their letters.

Test how the colors behave. In 2001, V.S. Ramachandran and Edward Hubbard ran four experiments with two synesthetes who saw colors with numbers and letters.

• The colors helped them see groups in a pattern, even though the black numbers alone did not group together.
• When numbers were shown off to the side of their vision, the colors did not appear.
• Roman numerals didn’t work. The colors needed the actual shape of the number.
• When two numbers took turns, the colors took turns too. Faster than four times a second, though, the colors were no longer there.

Their conclusion was that the colors are a genuine perceptual experience, not just a memory of what color goes with what.

What is happening in the brain?

Ramachandran and Hubbard suggested a simple idea they called cross-wiring. A part of the brain that handles color (area V4 or V8) and a part that handles the shapes of numbers both lie in the same region, the fusiform gyrus. If the two are more strongly linked than usual, seeing a number could switch on a color too.

In 2007, Romke Rouw and Steven Scholte put that idea to a test. They scanned 18 people with colored letters using diffusion tensor imaging, which looks at white matter, the long fibers that connect brain regions. Compared with matched non-synesthetes, the synesthetes had more coherent white matter. In one area of the temporal lobe, the difference was especially strong in projectors, the people who see the colors out on the page. That fits the picture of a brain with a few extra links between its circuits.

Does it run in families?

Often, yes. In 2009, Julian Asher and colleagues scanned the whole genome in 43 families, 196 people in all, where sound-color synesthesia showed up in several relatives. The strongest signal was on chromosome 2, with hints on chromosomes 5, 6 and 12. Earlier work had suggested a single gene on the X chromosome, but this study found no support for that, and it found two cases passed from father to son. The likely answer is several genes, which can differ from family to family.

In 2018, Amanda Tilot, Simon Fisher and colleagues studied three families with sound-color synesthesia across three or more generations. Each family had its own rare genetic variants, and no variant was shared between families. Six of the genes they highlighted help axons grow. Axons are the long fibers that let nerve cells connect. Those genes are active in early childhood, the time when synesthetic pairings form. Once again, the clues point to wiring.

Can a childhood toy pick the colors?

Here is a sweet surprise. In 2013, Nathan Witthoft and Jonathan Winawer reported 11 synesthetes whose letter colors were startlingly similar to one another. They traced the matches to childhood toys with colored letters.

In 2015, with David Eagleman, they looked at 6,588 English-speaking synesthetes. At least 400 of them, about 6%, had many letter colors that matched a widely available set of colored fridge-magnet letters. The toy was made between 1971 and 1990. For some five-year spans of birth years after it came out, the share approached 15%. Among people born five or more years before it was made, there were none.

The researchers were careful about what this means. The toy gave these people their colors. It did not give them synesthesia. One way to picture it: genes and wiring may set the stage, and the things a child sees while learning letters can choose the colors.

A little bit in all of us

Picture two shapes: a soft round blob and a spiky star. One is called “bouba” and the other “kiki.” Which is which?

If you said the round one is bouba, you have plenty of company. In a 2022 study, Aleksandra Ćwiek and colleagues played the two words to 917 people who spoke 25 languages using 10 writing systems. Across languages, the team estimated that about 72% matched bouba to the round shape and kiki to the spiky one. The effect held strongly across languages and did not depend much on how the words are written.

This isn’t synesthesia. Most people share it, and nobody actually sees a color. Researchers call it a crossmodal correspondence: a link between how something sounds and how something looks. Still, it’s a gentle reminder that our senses are always talking to each other.

In short

Synesthesia is when one sense brings an extra, automatic and steady experience, like colored letters, colored days or tasting words. It is far more common than people once thought, it shows up in children, and it can be checked with careful tests. Studies point to extra connections in the brain, genes that shape how nerve fibers grow, and even childhood toys that pick the colors. For most of us, the letter R is just a letter. For some, it is a little piece of sky.

#synesthesia #senses #perception #neural circuits #everyday brain

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