(I write more about synesthesia in my book Horror on the Brain.)

Rapper and hip-hop producer Pharrell Williams is a rare individual. Not just because he is a multiple RIAA-certified gold-record selling musician, and not just because he is one of the talented lyricists behind the rap duo N.E.R.D., but because he has a very unusual brain condition called synesthesia.

The majority of people don’t experience synesthesia. In these people, the major senses like sight, smells, and hearing, are all distinct from one another. However, for people with synesthesia, they experience a “crossing of sensations.” When one of their senses is activated, they may detect sensations at a different system. For example, the most common form of synesthesia is called grapheme-color synesthesia. Whereas a typical person would see a letter printed in black and white, a person with grapheme-color synesthesia may perceive that letter as being written in a color.

All sorts of senses, ranging from vision to emotions and even the passing of time can be subject to synesthesia. According to neurologist Richard Cytowic, there are more than 150 different types of synesthesia, including cases where specific ambient temperatures elicit a taste, or an emotional states may trigger smells. 1

The title of N.E.R.D.’s 2008 album, appropriately named Seeing Sounds, describes Pharrell’s particular type of synesthesia. In the introduction track, Pharrell describes the first time he experienced synesthesia:

“I’ll never forget, I was like seven years old. I closed my eyes, and that’s when it started. I started seeing sounds.”

Clinical estimates put the number of people with synesthesia somewhere between a low of .04% and as high as 25%.2 And for these people, the sensations of synesthesia are additive, meaning that senses are not replaced, but rather build on top of each other.

Synesthesia is technically a brain disorder. But, it is medically harmless with no adverse side effects. It’s not known to be associated with the onset of any other neurological disorders. And, it doesn’t indicate any underlying, latent neurological disease. Many synesthetes actually find the unusual sensations pleasurable!

(Fun aside about describing synesthesia during my sensation and perception lectures of intro neuroscience: In every class I’ve taught about synesthesia, I’ve always had at least one person in the class discover they were a synesthete. One student had number -> gender synesthesia, one was able to quickly identify the number “2” in a field of similar looking “5”s because of their number -> color synesthesia, and one regularly experienced sound -> shape synesthesia, just like Pharrell Williams.)

“But what about blind people? Can people who are blind see if they had synesthesia?”

One of my students in my most recent Introduction to Neuroscience class at DePaul University asked. If synesthesia is caused by some unusual, atypical wiring in the brain causing sense modalities to be activated in abnormal circumstances, can the visual cortex of a blind person become activated in the absence of receiving a visual input?

For people who lose their vision later in life, that is, past early childhood, their visual cortices have already developed much like that of a sighted person. During these early years when they had their vision, the light information from the eyes and optic tract that enter into the brain causes the visual cortical neurons to be wired similarly to that of a sighted person. The critical period, or time window for which it is important to get those signals, is probably only in the first few years of life for humans. Meaning, if a person is able to see during this formative period, the brain has already wired itself to gain visual information, and so these people are then able to interpret visual information even if they lose their eyesight later.

And so, it might be expected that late-blind individuals report some sorts of visual experiences when their synesthesia is triggered. One study described a handful of individuals who lost their vision due to a variety of conditions, ranging from traumatic injury to glaucoma. In the six cases described in the study, patients all observed some type of color-related synesthesia on receiving a stimulus, whether it be words, letters, or concepts. One of the individuals studied, Patient JF, would see rectangles of colors when thinking about a day of the week, even though he hasn’t been able to perceive color for 10 years. His color blindness was so severe, that he couldn’t tell you if a room were illuminated or not. A different late-blind person even reported that particular combinations of Braille dots, which collectively represent a letter, triggered the sensation of color: a nearly-perfect correlate of the oh-so-common grapheme-color synesthesia that is reported in as high as 60% of sighted synesthetes! 3

Now, what about if a person had never experience vision at all? Would they still see some sort of visual cortex activity in their synesthetic experiences? Researchers from the University of Bath in the UK conducted a study on a congenitally blind man who lost his eyesight during birth. He was not born a synesthete, but only experienced the crossing of sense modalities after taking LSD. In his words:

“Sound, touch and smell were experienced at the same time, whenever I took any drug, but it was always the sounds that played a big, big part. Probably the most important part during all of my trip experiences. I guess my auditory experiences just did it for me!”4

Although imaging studies were not conducted on the patient’s brain, I predict that areas of his visual cortex would show increased activation. For congenitally blind individuals, the visual cortex may encode non-visual stimuli. In particular, the occipital lobe may carry information related to sound - this phenomenon has been demonstrated among blind individuals who use echolocation as a means of navigation.5 And in this case, the person is not experiencing sight, but is still perceiving visual inputs - just not the same visual inputs that a sighted individual would experience.

There is another confounding variable that arises with studying congenitally blind synesthetes. People without knowledge of the visual system may also lack the language to describe vision. For describing shapes, they may be just as good as sighted individuals. Using the sense of touch, they have experienced rectangles and circles, and can likely explain these shapes even without ever using their eyes. But blind individuals are missing the concept of colors. For sighted individuals, color words are nearly universal. 490 nm wavelength light is almost always called a shade of blue, while 700 nm wavelength light is always some kind of red. People who have never experienced color vision will have no concept of these colors, and would therefore have a difficult time explaining them even if their synesthesia allowed for perception of colors. Because they are missing the semantic framework related to color vision, self report becomes almost impossible.

At the end, it comes down to the definition of “see.” For sighted people, or for people with knowledge of vision such as a person who loses their vision late in life, seeing implies a use of the eyes. But for people who have been blind since birth, their capacity to “see” may rely on the use of different senses.

1. Wednesday is indigo blue: discovering the brain of synesthesia

2. Synaesthesia: The prevalence of atypical cross-modal experiences

3. Visual synaesthesia in the blind.

4. Synesthetic hallucinations induced by psychedelic drugs in a congenitally blind man

5. Neural Correlates of Natural Human Echolocation in Early and Late Blind Echolocation Experts