What is synesthesia? When senses blend together

Synesthesia is a trait in which experiencing one thing automatically and consistently triggers an extra, unrelated experience. For someone with synesthesia, the letter A might always be red, a trumpet might look orange, or a word might bring a taste. The name comes from Greek words meaning "perceiving together". It is not an illness: it is a difference in how a brain is wired, found in at least 4% of people, and most who have it consider it simply how the world is.

In short

• In synesthesia a trigger, such as a letter, a sound or a day of the week, reliably brings an extra experience, such as a color, a shape or a taste.
• Scientists confirm it by consistency: a synesthete picks almost the same color for each letter months apart, which others can't do from memory.
• It runs in families and seems to reflect extra or differently tuned connections between brain areas, and it is a natural variation, not a disorder.

What does synesthesia feel like?

The trigger is called the inducer, and the extra experience the concurrent. A synesthete reading the word "seven" might see it tinged with green, or simply know, with the same certainty as knowing its meaning, that seven is green. Some see the colors out in the world, projected onto the page. More, usually, experience them in the mind's eye. Either way, the experience is automatic. It can't be switched off, and it doesn't need any effort.

The pairings are personal. Two synesthetes rarely agree on the color of A, though some patterns are common: A is often red, O often white. They are also stable. A synesthete whose Tuesday is a pale yellow at 8 will usually find it pale yellow at 80. The novelist Vladimir Nabokov described his colored alphabet in his memoir Speak, Memory, and was surprised to find his mother had one too, with different colors.

What are the main types?

At least 60 types have been described. Some of the best known, drawing on the Cleveland Clinic's overview:

Type	The trigger	The extra experience
Grapheme-color	Letters and numbers	A color for each
Colored days and months	Days of the week, months	A color for each
Sound-color, or chromesthesia	Music, voices, noises	Colors, sometimes shapes and movement
Time-space, or sequence-space	Numbers, dates, months	A position in space, such as a year laid out as a loop around the body
Lexical-gustatory	Words, or speech sounds	A taste in the mouth
Mirror-touch	Seeing someone else being touched	Feeling the touch on one's own body
Personification	Letters, numbers, days	A gender or personality, such as a bossy 4 or a shy Thursday

Many synesthetes have more than one type. Not every blending of the senses counts: saying a cheese is "sharp" or a sound is "bright" is ordinary metaphor that everyone shares. Synesthesia is specific to the person, automatic and consistent.

How do scientists know it's real?

For a long time synesthesia was dismissed as imagination, vivid metaphor or childhood memories, such as a set of colored fridge magnets. The key test is consistency over time. Anyone can say A is red when asked. Few people can give the same exact shade for 36 letters and digits, shuffled, without being able to remember what they said before.

How the consistency test works

The standard online test, the Synesthesia Battery, developed by David Eagleman and colleagues, works like this, as described in a 2015 validation study:

1. The letters A to Z and digits 0 to 9 appear one at a time on screen, in random order, each three times: 108 trials in all.
2. Each time, the person picks the color that goes with it from a large color palette, or marks it as having none.
3. For each letter, the test measures how far apart the three chosen colors are in color space, and averages that across all the letters.
4. A synesthete picking the same red for A all three times scores close to zero. Someone guessing or relying on memory spreads widely. A score below 1 counts as synesthesia, though some researchers argue for a looser cut-off of 1.43.

When the researchers tested 2,847 randomly recruited people, 140 said they had colored letters or digits, but only 34 passed the consistency test, about 1.2% of the sample. Most people who think they have synesthesia don't, by this measure. The original approach was slower: in earlier studies, people were simply asked again after months without warning, and genuine synesthetes still matched themselves closely.

Other tests show the colors work like perception, not like memory. In a 2001 study, the neuroscientists V. S. Ramachandran and Edward Hubbard hid a shape made of 2s among a field of 5s, which look alike in a plain font. Their two grapheme-color synesthetes, who saw 2s and 5s as different colors, found the hidden shape more readily than non-synesthetes. Brain scans also show color areas becoming active when synesthetes look at black letters.

How common is it?

Estimates depend on the type and on how carefully it is tested. An influential survey led by Julia Simner in 2006 sampled people without asking for volunteers and used objective tests. It put the overall figure at around 4%, and found colored days of the week to be the most common form. Grapheme-color synesthesia is about 1% to 2%, depending on the cut-off. Some forms, such as time-space, may be far more common, perhaps one person in eight by some estimates cited by the Cleveland Clinic, though that figure is less firm. Because many synesthetes assume everyone experiences the world as they do, a lot of them never know they have it.

What causes synesthesia?

It tends to run in families, and a 2018 study by Amanda Tilot, Simon Fisher and colleagues at the Max Planck Institute for Psycholinguistics sequenced the DNA of three families with sound-color synesthesia across several generations. Each family had different rare gene variants, but they clustered in genes involved in growing axons, the long fibers that link brain areas. Relatives often have different types, so what is inherited seems to be a tendency, not a specific pairing.

The leading explanation is cross-activation. In the brain's fusiform gyrus, the area that recognizes letter and number shapes sits right next to an area that processes color. Ramachandran and Hubbard proposed that in grapheme-color synesthetes, extra or unpruned connections between the two make activity in one spill into the other. Babies' brains have many more connections than adults', and most are pruned away in childhood, so synesthesia may be what happens when some survive. An alternative view holds that the connections are ordinary but feedback from higher areas is less inhibited. Most researchers think both may play a part. Synesthesia can also appear later in life after brain injury, blindness, or with hallucinogenic drugs, but that acquired form tends to be less consistent.

What does synesthesia reveal about perception?

Synesthesia shows that perception is built, not received. Light of a certain wavelength doesn't contain "red". The brain constructs the color, and in synesthesia it constructs one where the input has none. That makes it a natural experiment in how the brain turns signals into experience.

It also hints that everyone is a little synesthetic. Asked which of two shapes, one rounded and one spiky, is called "bouba" and which "kiki", most people in many languages call the rounded one bouba. Mappings like that may be the faint, shared version of what synesthetes experience strongly.

Synesthetes tend to have some advantages, such as better memory for things that trigger their colors, and some studies suggest they are more common among artists. Few want to lose it. It is studied by psychology and neuroscience together, alongside other inner experiences that are hard to verify from outside, such as dreams.

Check yourself

• Without looking back: why is consistency over time the main evidence that synesthesia is real and not imagination?
• How does cross-activation explain grapheme-color synesthesia, and which brain areas does it involve?
• What is the difference between synesthesia and an ordinary metaphor like "a loud shirt"?

Sources

• Synesthesia — Cleveland Clinic
• Validating a standardised test battery for synesthesia: Does the Synesthesia Battery reliably detect synesthesia? — Carmichael et al., Consciousness and Cognition, 2015
• Psychophysical investigations into the neural basis of synaesthesia — Ramachandran and Hubbard, Proceedings of the Royal Society B, 2001
• Seeing sounds: researchers uncover molecular clues for synaesthesia — Max Planck Institute for Psycholinguistics, 2018