Some people don’t just listen to music — they taste it, smell it, see it, and may even be tickled by it. Having a stimulus trigger more than one sense is a neurological condition known as synesthesia. Let’s dive into the fascinating world of people who can literally taste the rainbow. Make way, Skittles.

What is synesthesia?

This extraordinary sensory condition, whose name means “to perceive together” in Greek, is shared by around one in 2,000 people and comes in many forms and varieties. There are up to six times more females with synesthesia than there are males.

Some people with synesthesia, called synesthetes, taste shapes, others perceive written digits, letters, and words in colors, and some see abstract concepts, such as time and mathematical operators, as shapes projected in the space around them. It’s common for synesthetes to share more than one form of multi-sensory perception. Crucially, synesthesia is remarkably consistent, vivid, and automatic. For instance, someone who sees the number “8” as orange will do so nearly every time, unlike regular people who might only imagine a color — and they can’t help it either. Synesthesia isn’t something you can switch on and off at your choosing.

Although some synesthetes can become exhausted sometimes from the sensory overload, the condition is not generally associated with any health or cognitive problems. In fact, most synesthetes treasure and adore their ability, treating it as a gift, although in the beginning, it may have been challenging to understand it as they slowly learned they experienced the world very differently from other people.

Many artists, writers, musicians, and scientists are synesthetic, perhaps because this condition may help them think more laterally and creatively. Some examples include writer Vladimir Nabokov, painter Wassily Kandinsky, singers Tori Amos and Pharrell Williams, composer Franz Liszt, and physicist Richard Feynman.

Although a lot of progress has been made in demystifying synesthesia in recent years, largely thanks to brain imaging studies, there is much we don’t know about this neurological condition, including its origin.

The different tastes and flavors of synesthesia

Since there are five senses — sight, hearing, taste, touch, and smell — and synesthesia is generally defined as the crossover of two senses or perceptions, there are numerous possible combinations. There may be a staggering 80 or more types of synesthesia, each targeting a different combination of senses and stimuli. However, some types of synesthesia are much more common than others. Grapheme-color synesthesia, or seeing letters in color, appears to be one of the most common ones, whereas word-gustatory synesthesia, which involves tasting words, is far less often reported.

Some forms of synesthesia are easier to describe and envision. For instance, virtually anyone can imagine what it could feel like to see colored letters, numbers, and words, whereas, for a person that can hear sounds attached to every color, the experience is not at all straightforward to describe.

Synesthetes can be classed into two major groups. First, there is projective synesthesia where one of the five senses triggers a second one by the initial stimulus. One example would be tasting apples when hearing a certain musical note or specific timbre like a guitar.

The second major group is associative synesthesia, occurring when someone associates the stimulus very strongly with a color or shape, but does not literally experience it like in projective synesthesia. For instance, someone with grapheme-color synesthesia might see the number 6 as blue, whereas someone with associative synesthesia might strongly feel that the number 6 is shy and is anxious to sit next to 7. Others might feel that the letter “B” is optimistic or generous.

Some of the most common or interesting forms of synesthesia include:

• Grapheme-color synesthesia. This is probably the most prevalent and well-studied form of the condition. In it, synesthetes commonly associate written letters and numerals with colors. Some researchers have suggested that this form of synesthesia is owed to “cross-writing” between the distinct centers of the brain responsible for processing colors and numbers, respectively, both of which are located in the same brain region known as the fusiform gyrus. People with grapheme-color synesthesia often also have a good memory, using it as a mnemonic device to remember things more easily.
• Chromesthesia. Also known as sound-to-color synesthesia, this particular type of synesthesia can be simply described as seeing sounds. Certain sounds are perceived as a particular color, which is experienced constantly every time that sound is heard. High-pitched sounds are often associated with light, bright colors, while low-pitched sounds like bass are most frequently matched to darker colors. Some individuals with chromesthesia have their ability evoked only when listening to musical notes, while in others the synesthesia is triggered by all manners of sounds.
• Spatial-sequence synesthesia. This is one of the most peculiar and particularly fascinating synesthesias, involving perceiving sequences like numbers and letters, but most often the months of the year and dates, as occupying points in space. If I asked you where exactly in the room is six o’clock without pointing at a clock, you’d probably give me a suspicious stare, but this question is perfectly reasonable for people with spatial-sequence synesthesia. In fact, the entire span of the clock extends around them like a ring, each hour situated in its own place on the ring.
  

• Mirror-touch synesthesia. Empathy is said to be a defining human quality, but people with mirror-touch synesthesia go above and beyond, having the ability to literally feel the same sensations as somebody else. For instance, seeing a doctor placing a cold stethoscope on somebody’s back, whether in the hospital or on a device’s screen, will make their own back feel cold. As you might imagine, for these individuals, watching a violent horror movie may be out of the question.
• Auditory-tactile synesthesia. While seeing sounds is more common, touching sounds is one of the rarer synesthesias. Studies suggest that the condition can be explained by cross-wiring between auditory and somatosensory cortices of the brain. While listening to music we like can provoke goosebumps, people with auditory-tactile synesthesia feel sounds as touch in a manner more akin to an everyday experience. Hearing a certain sound may feel like tingling, tickling, or having something press against the skin. Many times, sounds are described as warm or cold. There are also forms of synesthesia when a tactile sensation is associated with another sense, such as sight. For instance, some people can see the color of a feather brushing their necks.Lexical- and sound-gustatory synesthesia. People with lexical-gustatory synesthesia experience certain words, whether spoken or written, as distinct tastes, smells, and textures in their mouth, while sound-gustatory synesthesia involves the same experience but is triggered by sounds.

What are the symptoms of synesthesia?

Since there are so many different types of synesthesia, it can be challenging to list the exact symptoms, amplified by the fact that you can experience more than one type of synesthesia at the same time. However, the most symptoms include:

• an uncontrollable perception of sensing more than one sense at the same time.
• the synesthetic perceptions are consistent, such as seeing the letter D always colored in orange.
• difficulties describing the unusual perceptions to other people.

There is no treatment for synesthesia, partly because there is little interest in one. Anecdotally, most people with anesthesia seem to enjoy their ability to experience the world in a totally different way from everybody else.

Why some people are synesthetes

It’s not clear what causes synesthesia, but the evidence so far suggests genetics play a major role. Although synesthesia is very rare among the general population, over 40% of synesthetes have a parent, sibling, or offspring who also shares the condition. For instance, writer Vladimir Nabokov, his wife, and his son, all had synesthesia.

“[W]e discovered one day that my son […] sees letters in colors, too. Then we asked him to list his colors and we discovered that in one case, one letter which he sees as purple, or perhaps mauve, is pink to me and blue to my wife. This is the letter M. So the combination of pink and blue makes lilac in his case. Which is as if genes were painting in aquarelle.”

Vladimir Nabokov speaking to the BBC in 1962.

While most cases of this condition are congenital, there is recent evidence that synesthesia can be acquired in certain conditions, such as when triggered by a psychoactive drug or even through experience. One study, for instance, found that some US veterans undergoing treatment in a Pennsylvania hospital for PTSD had acquired grapheme-color synesthesia. But although there are cases of synesthesia occurring following brain damage, drug abuse, or sensory loss (i.e. going blind or deaf in adulthood), most synesthetes report symptoms for as long as they can remember.

Although scientists can’t tell for sure why synesthesia exists, cross-activation and disinhibited feedback are the two leading hypotheses.

The cross-activation hypothesis posits that synesthesia arises due to an incomplete ‘cleanup’ of neurons early in life. Neurons talk to each other through connections called synapses. Some form strong connections, while other synapses are weaker, and through a process known as synaptic plasticity, the brain wants to prune these weaker and seldom used synapses. This process largely happens during infancy. In people with synesthesia, some scientists claim, this pruning among connections formed between neurons responsible for different senses is incomplete, allowing one sense to trigger another. The implication of this theory is that we are all born with synesthesia-like abilities as infants, but lose them as we age.

The disinhibition theory of synesthesia posits that in synesthetes, adjacent brain regions that are responsible for processing and interpreting different senses — and which are normally insulated from each other — are concurrently activated in the same pathways, triggering the sensation of two or more senses from just a single type of stimuli.

Having synesthesia is generally safe and, for some, even entertaining. In some rare cases, though, there is evidence that suggests space-time synesthetes, in particular, are more prone to develop Hyperthesmic Syndrome, which involves an inability to forget even the smallest details about the events of one’s life. Other types of synesthesia, however, haven’t been associated yet with health problems.

If you think you might be synesthetic, there’s an online questionnaire you can complete to find out here.