A select group of synesthetes can truly "taste the rainbow."

Of all the phenomena of the brain, synesthesia is one that’s equally fascinating and mysterious. Characterised by senses essentially overlapping (i.e. hearing and vision, or hearing and taste), synesthesia is a neurological phenomenon in which information meant to stimulate one of your senses stimulates several of your senses

“People who have synesthesia are called ‘synesthetes,'” explains neurologist Romila “Romie” Mushtaq, M.D., ABIHM., adding that the word synesthesia comes from the Greek words synth, which means together, and ethesia, which means perception.

Sleep and neurology expert Chris Winter, M.D. says synesthesia is ultimately the sensory perception of experiencing a crossing over of your senses. “Certain words might elicit something within an individual that really has nothing to do with the word,” he says. For example, someone hearing the word bounce and seeing the colour yellow in their mind’s eye.

“Synesthetes can often see music as colours when they hear it, and taste textures, describing them as round or pointy when they eat. Other example of synesthesia are seeing the same colour every time you see a certain number, or hearing sounds with light motion,” Mushtaq notes.

How Common Is It?

According to Mushtaq, synesthesia is evident in roughly 3 to 5% of the population, and in about 40% of those people it’s genetic. It’s also significantly more common in women than men.

“Synesthesia is not considered a disease, nor is it associated with a higher rate of mental disorders,” she explains, adding “Books and articles in the past have eluded to synesthesia being related to mental health disease, but research shows that there is no correlation between synesthesia and schizophrenia, psychosis, addiction, or any other mental health disease.”

There’s also no clinical diagnosis for synesthesia.

One link that has been found in research however, is synesthesia and autism. In one 2020 study, researchers note that there may be higher rates of synesthesia among diagnosed individuals with autism. “Up to 20% of autistic individuals may experience synesthesia, and more research is currently being conducted to understand the correlation between the two,” Mushtaq says.

Signs A Person Has Synesthesia

First, we’ll break down a few common ways people experience synesthesia, though it’s important to note synesthetes will experience their synesthesia differently, and may experience varied combinations of these signs.

  • Seeing colours when you hear music
  • Tasting something when you hear certain words
  • Seeing colours when you see certain letters or numbers
  • Feeling touch or other sensations when you see someone else experiencing that touch
  • Feeling a physical sensation when you hear certain sounds
  • Smelling something when you hear certain sounds
  • Seeing colours when you smell something

On top of the way synesthesia presents for individuals, there are some other signs that indicate someone really is a synesthete. For one thing, with synesthesia, the sensory experiences are consistent (i.e. if the number seven looks green to a synesthete, it will always be green, even years later).

Additionally, synesthesia happens unwillingly without having to control or even think about it.

What Causes Synesthesia?

The jury is ultimately still out on what causes synesthesia, with Mushtaq noting that it’s not well understood. As aforementioned, synesthesia may be due to genetics in about 40% of the people, who were simply born that way.

“In genetic cases, we see the brain has enhanced connections between different regions of the brain cortex associated with senses,” Mushtaq adds. “There are also cases of synesthesia that have been reported that had a sudden onset in adulthood, for example, caused by hypnosis, drug exposure to psychedelics, or traumatic brain injury.”

And according to Winter, it’s especially difficult to understand because it presents so differently among individuals. Some may experience synesthesia in a variety of ways, while some may only experience it one way. “Whatever it is, it could be on a continuum or a spectrum,” he says, adding that it’s important to remember it is not a pathology.

How To Test For It

If you’re curious to see whether you could have synesthesia, Mushtaq says a neuropsychologist or neurologist can administer the Synesthesia Battery Test, which is 80 questions and takes about 20 minutes.

You can also find tests online that basically ask you to associate a colour you see with a number, and then you are retested to see if the colours remain the same.

For a quick test you can do yourself, the same principles apply. Try to recruit a friend to track your responses, or log them somewhere you won’t look at them.

  1. Ask yourself to identify the colours of letters, numbers, weekdays, and months.
  2. After a couple of weeks, repeat the same questions.
  3. If you name the same (or at least a similar) colours for a given letter, number etc., that would be a sign you might have synesthesia.

The Research

More and more research is being done to understand synesthesia (with the primary methodology being neuroimaging) in order to look at what’s really happening in the brain and nervous system of those who have synesthesia. These studies indicate that those who inherited their synesthesia may share over 30 rare genetic variants, though researchers aren’t sure whether synesthesia could have posed some evolutionary advantage in the past.

And while there is currently no treatment for synesthesia, there doesn’t really need to be, as it’s not a disease or disorder.

For most synesthetes, in fact, the experience isn’t a bad thing. For some people, it can even be advantageous, such as artists who can paint what a song looks like to them.