The Tell-Tale Brain: A Neuroscientist's Quest for What Makes Us Human

A second explanation that is sometimes posed is that synesthetes are just remembering childhood memories and associations. Maybe they played with refrigerator magnets, and the 5 was red and the 6 was green. Maybe they remember this association vividly, just as you might recall the smell of a rose, the taste of Marmite or curry, or the trill of a robin in the spring. Of course, this theory doesn’t explain why only some people remain stuck with such vivid sensory memories. I certainly don’t see colors when looking at numbers or listening to tones, and I doubt whether you do either. While I might think of cold when I look at a picture of an ice cube, I certainly don’t feel it, no matter how many childhood experiences I may have had with ice and snow. I might say that I feel warm and fuzzy when stroking a cat, but I would never say touching metal makes me feel jealous.

A third hypothesis is that synesthetes are using vague tangential speech or metaphors when they speak of C-major being red or chicken tasting pointy, just as you and I speak of a “loud” shirt or “sharp” cheddar cheese. Cheese is, after all, soft to touch, so what do you mean when you say it is sharp? Sharp and dull are tactile adjectives, so why do you apply them without hesitation to the taste of cheese? Our ordinary language is replete with synesthetic metaphors—hot babe, flat taste, tastefully dressed—so maybe synesthetes are just especially gifted in this regard. But there is a serious problem with this explanation. We don’t have the foggiest idea of how metaphors work or how they are represented in the brain. The notion that synesthesia is just metaphor illustrates one of the classic pitfalls in science—trying to explain one mystery (synesthesia) in terms of another (metaphor).

What I propose, instead, is to turn the problem on its head and suggest the very opposite. I suggest that synesthesia is a concrete sensory process whose neural basis we can uncover, and that the explanation might in turn provide clues for solving the deeper question of how metaphors are represented in the brain and how we evolved the capacity to entertain them in the first place. This doesn’t imply that metaphor is just a form of synesthesia; only that understanding the neural basis of the latter can help illuminate the former. So when I resolved to do my own investigation of synesthesia, my first goal was to establish whether it was a genuine sensory experience.

IN 1997 A doctoral student in my lab, Ed Hubbard, and I set out to find some synesthetes to begin our investigations. But how? According to most published surveys, the incidence was anywhere from one in a thousand to one in ten thousand. That fall I was lecturing to an undergraduate class of three hundred students. Maybe we’d get lucky. So we made an announcement:

“Certain otherwise normal people claim they see sounds, or that certain numbers always evoke certain colors,” we told the class. “If any one of you experiences this, please raise your hands.”

To our disappointment, not a single hand went up. But later that day, as I was chatting with Ed in my office, two students knocked on the door. One of them, Susan, had striking blue eyes, streaks of red dye in her blonde ringlets, a silver ring in her belly button and an enormous skateboard. She said to us, “I’m one of those people you talked about in class, Dr. Ramachandran. I didn’t raise my hand because I didn’t want people to think I was weird or something. I didn’t even know that there were others like me or that the condition had a name.”

Ed and I looked at each other, pleasantly surprised. We asked the other student to come back later, and waved Susan into a chair. She leaned the skateboard against the wall and sat down.

“How long have you experienced this?” I asked.

“Oh, from early childhood. But I didn’t really pay much attention to it at that time, I suppose. But then it gradually dawned on me that it was really odd, and I didn’t discuss it with anyone…I didn’t want people thinking I was crazy or something. Until you mentioned it in class, I didn’t know that it had a name. What did you call it, syn…es…something that rhymes with anesthesia?”

“It’s called synesthesia,” I said. “Susan, I want you to describe your experiences to me in detail. Our lab has a special interest in it. What exactly do you experience?”

“When I see certain numbers, I always see specific colors. The number 5 is always a specific shade of dull red, 3 is blue, 7 is bright blood red, 8 is yellow, and 9 is chartreuse.”

I grabbed a felt pen and pad that were on the table and drew a big 7.

“What do you see?”

“Well, it’s not a very clean 7. But it looks red…I told you that.”

“Now I want you to think carefully before you answer this question. Do you actually see the red? Or does it just make you think of red or make you visualize red…like a memory image. For example, when I hear the word ‘Cinderella,’ I think of a young girl or of pumpkins or coaches. Is it like that? Or do you literally see the color?”