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Crossed wires: what is synesthesia, and how does it affect the brain?

Updated: Feb 4, 2023

Give a synesthete a list of concepts, and they will respond with a seemingly random group of sensory experiences: The number six might be lavender, but their name is definitely emerald green. Seeing a close friend could produce a lemony flavor. Their favorite song appears as wavy orange lines, while a family member’s voice appears as colorful shapes across their face.

If this sounds familiar, you too may have the condition known as synesthesia — literally “together” and “perception” in Greek. It’s described by Kathryn Watson for as “a neurological condition in which information meant to stimulate one of your senses stimulates several [others],” producing an experience of the world in which the senses bleed into one another.

Most with the condition were born with it, or had it become apparent as young children. It can be induced through psychedelic substances (such as LSD), but true synesthesia begins in early childhood, and may be hereditary. It is important to note, too, that the experiences that come with it are involuntary, and that no two synesthetes’ experience is the same. For example, I may associate the number three with the color turquoise, while you may associate it with bright pink. In addition, synesthesia is characterized by a consistency over time, meaning that the flavor you ascribe to a certain piece of music will show little variation from when you listened to it last year, to when you heard it today.

Synesthesia appears in a variety of ways, meaning that its prevalence is estimated at anywhere from one in 25 to one in 20,000 people, depending on how it is defined. Some of the most common types are grapheme-color and sound-to-color, though as many as eighty varieties have been identified.

Grapheme-color synesthesia is found in about one percent of the population. According to Dr. Devin Blair Terhune in an article for the New York Times, “We all associate numbers and colors to some extent,” but in the case of grapheme-color synesthetes, characters like numbers and letters are consistently connected to the same colors. For example, someone may read the letter “a” written in black, but to them it will possess a quality of, say, gold, every time they see it. What grapheme-color boils down to, says Dr. Terhune, is a “lower threshold” for visual stimulation. In a study conducted by the University of Oxford, it was found that grapheme-color synesthetes require only one third of the visual cortex stimulation of non-synesthetes to trigger the same visual experiences.

Sound-to-color synesthesia (known scientifically as “chromesthesia”) is another of the more well-known forms, appearing in about one in every 3,000 individuals. With this variety, experiencing sounds -- music especially -- will trigger a visual response. Such responses may include moving or stationary colors that appear in the mind's eye, which are usually consistent over time. Non-synesthetes can create a similar experience by playing a song or piece of music, and picking the colors that they feel are representative of it. The key difference between this and the experience of a synesthete is that the latter is involuntary. While one without the condition can produce a beautiful array of colors by focusing on the mood, key, and tempo of the music, a synesthete’s experience is entirely unsolicited, and can, at times, be distracting.

Synesthesia may seem like a neurological condition that’s been researched only recently, but there is evidence of its traits appearing in writers, artists, musicians, and philosophers going back thousands of years. The question is — given that the condition is inherited — why did millennia of human evolution retain this genetic quirk?

As Jennie Cohen puts it in an article for, “With all the challenges they faced as they ascended the food chain, early humans surely had little need for seeing blue when a bird chirped or for tasting berries when they touched damp leaves.” In addition, synesthesia can cause certain impediments, like confusing locations because you associate them with the same color, or having an instant aversion to someone because their personality has a flavor that you don’t like -- both of which could hinder the survival of prehistoric humans. However, synesthesia also comes with a host of benefits that would have aided our ancestors, which may account for its resistance to being factored out of natural selection. For example, synesthetes are more likely to be better at perceiving the world around them, and those who associate colors with numbers and shapes may have been useful when remembering which plants and animals contain poison or venom, and which are edible or docile. Also, the condition has been linked to heightened levels of creativity, because who doesn’t want a bard around when migrating from one region to another?

Greek philosopher and mathematician Pythagoras wrote the first description of synesthesia around 500 B.C. According to the American Psychological Association (APA), “In the late nineteenth and early twentieth centuries, synesthesia enjoyed a flurry of scientific study, mostly descriptive.” The condition had another spike in interest in 1987, when Simon Baron-Cohen, PhD, and a team of researchers conducted an experiment on a synesthete and several non-synesthetes. Both were asked to describe the color that they associated with a certain word, and then were asked to perform the same activity a year later. The non-synesthetes’ responses in the second phase of the experiment had about 20 percent similarity to their earlier responses, while the synesthete had over 90 percent consistency. This experiment was “the first hard evidence that synesthetes' experiences are consistent across time.”

In the year 2000, another experiment was conducted by researchers Mike Dixon, PhD, Daniel Smilek, Cera Cudahy and Philip Merikle, PhD, which proved, by showing a grapheme-color synesthete simple math equations, that the sensory experience produced by a character could be caused by just its concept. For example, when the experiment’s subject added two and five, they experienced the color of seven, even though the number was not physically visible.

What is still unclear about synesthesia, however, is what causes it within the brain. In the 1900s, “researchers ascribed synesthesia, somewhat vaguely, to ‘crossed wires’ in the brain,” says the APA. Currently, though, there are two leading hypotheses.

The first states that synesthetes possess more neural connections than their non-synesthete counterparts. This is a more scientifically advanced version of the “crossed wires” theory, and describes the idea that the synesthetic mind is hyper-connected, meaning that one’s literal experience of hearing C sharp on the piano sends a message from the auditory part of the brain to the visual part, triggering the visual perception of the color emerald green. Interestingly, an extension of the hyper-connectedness theory proposes that we are all born with the neural connections that allow for synesthesia, but that most shed them as they grow older.

The second theory, on the other hand, states that synesthetes have the same amount of neural connectedness as anyone else, but that their sensory experiences become jumbled on the way from multi-sensory areas of the brain to single-sensory ones. If you think about the brain like a hotel, the perception of a friend’s voice for a non-synesthete enters into the lobby with all the other sensory inputs, and then takes its elevator up to where it’s staying on the “sound” floor. But for a synesthete, the friend’s voice enters the lobby, presses the wrong button on the elevator and finds itself on the “taste” floor -- where it spends a while looking for its room, and then realizes that it has to go back and take another elevator to get to where it belongs on the “sound” floor. This theory also accounts for the fact that synesthesia can be temporarily induced through psychedelic substances, which cause the brain’s inputs to get jumbled and end up on the wrong floors.

More visibility of the condition may lead to more studies to locate its cause. Synesthesia has been linked to an abundance of creativity, including among famous musicians like Billie Eilish. “I think visually first with everything I do,” Eilish says. “I have synesthesia, so everything that I make I'm already thinking of what color it is, and what texture it is, and what day of the week it is, and what number it is, and what shape.”

Synesthesia is a neurological condition, as opposed to an illness, and most who have it wouldn’t want it removed, even if there were a cure. Losing synesthesia would be akin to losing the ability to see in color -- for some, it’s how the world is perceived, and its upsides are well worth the drawbacks. The more people learn about conditions like synesthesia, the more we can understand them, ourselves, and each other.

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