Synesthesia: When Senses Collide

Like swimmers and polite, non-city drivers, our senses of sight, sound, touch, smell, and taste usually stay in their own lanes. Each sense takes its own path up to the brain, refusing distraction by the jumble of other travelers. But every so often, a driver swerves, the paths criss-cross and blur—and that’s when we wind up with an individual who can hear a taste or see a smell. That ability is called synesthesia. 

Synesthesia is a neurological phenomenon (much too cool to call a disorder) in which people experience one sense and simultaneously activate another. To someone with synesthesia, touching wool might also trigger the perception of the color green, the sound of a plane overhead, or the smell of vanilla. A synesthete—someone with synesthesia—might even see the letter A and also think of a specific personality trait (“that A looks awfully narcissistic…”). The most common way to experience synesthesia is to look at letters or numbers and perceive them as having a specific color, but there are as many types of synesthesia as there are facets of our senses and ways to mix them up. 

How does it happen?

Science is still on the case, but synesthesia is most likely caused by extra connections among the pathways of the senses. So if you see the letter B as orange, there would be a place in your letter-perception stream that leaks into your color-perception stream, causing you to immediately and involuntarily perceive orange when you see a B (often a very specific shade of orange). Good for you—reading must be pretty psychedelic.

But how do the paths veer off course in the first place? Well, although some people experience synesthesia temporarily through deep meditation, sensory deprivation, or the 1960s (more specifically, LSD), most people with synesthesia are born with it. It’s believed to be genetic, perhaps affecting as much as 2 percent of the population. 

Scientists think synesthetes must lack the sort of brain “pruning” process that keeps most of our senses neatly divided. When our brains are newly forming in utero and in early life, they’re actually quite interconnected. Lots of different areas talk to lots of other areas, like a college campus on orientation weekend. But over time, the pathways in our brains get more efficient, hone in on their roles, and slowly stop talking to people not involved (think art kids with art kids and pre-meds with pre-meds). Little by little, those other connections are snipped away by the grand neural gardener until only the essentials remain. But for people with synesthesia, the pruner missed a spot. Groups that should be separate can still talk to each other.

What synesthesia feels like

People with synesthesia often grow up without realizing there’s anything abnormal about them. The rogue associations come so naturally and automatically that synesthetes they might not figure out what’s going on until they ask, “Isn’t G such a pretty color?” and get some strange looks in response. Associations usually stay consistent over time—once an orange B, always an orange B—but they aren’t entirely random. For example, among people who see letters in color, A is far more likely to look red than any other color. Nobody’s exactly sure why.

To get a sense of how it works, consider a famous test put together by superstar neurologist V.S. Ramachandran. A person is shown a sheet of paper with lots of number 5s, plus a few number 2s scattered throughout to form a triangle. If you aren’t a number-to-color synesthete (and we’re assuming most of you reading this post are not), it looks like this: 

A bunch of 5s, or a bunch of 5s with some 2s thrown in. A synesthete, on the other hand, will tell you right away that they see a triangle. Because they process 2s in a different color than 5s, the 2s veritably pop off the page, and the shape is easy to recognize. Like this:

And if that didn’t blow your mind, you must be a synesthete.

Perfume and synesthesia

When they’re not identifying triangles, synesthetes are often artists. Vladimir Nabokov (author of Lolita) saw letters and sounds as colors, and Billy Joel hears music in color, even using it to create songs.

It isn’t entirely uncommon to find a perfumer with a touch of synesthesia, either. (See our interview with Dawn Spencer Hurwitz for an account.) Scent most commonly triggers a corresponding color, but it can also lead to tastes, sounds, or images. Some synesthete perfumers use their abilities to create scents that not only smell but sound, look, or taste beautiful. (But only in their minds—please stick to a strict “don’t taste your perfume” policy.)

A touch of synesthesia

Some would say that we all have a touch of synesthesia. Take the bouba/kiki effect. People are shown a round shape and an angular shape, then asked to name one of them “kiki” and the other “bouba.” Upwards of 95 percent of people name the angular shape kiki and the round one bouba. The results even held up when the experiment was conducted with Tamil speakers in India. There’s something about the way the words sound that makes us want to attribute a certain shape to them. So even for us non-synesthetes, our senses may be itching to cross the lanes.

We often see this effect in perfume writing, too. Perhaps it’s inevitable that describing a scent in words will lead to some synesthetic mash-ups. Sometimes these tend to be universal—most would describe galbanum, an aromatic resin, as smelling green, and many will probably imagine warm brown tones when smelling vanilla. Sometimes, though, our descriptions of scent tend to be too reliant on memory to be considered synesthesia—or even to be considered universal. (See this post for more on why and how smells are linked to personal memories in our brains.)

What about you? Do you experience any sort of synesthesia? Tell us about it in the comments. That way, the rest of us can be jealous of your gorgeous orange Bs and delicious 5s.