I recently read Wednesday is Indigo Blue by Richard Cytowic and David Eagleman. The book explores the psychological phenomenon of synesthesia, a hot topic. Whether or not you recognize the word, you’ve probably heard stories about people who could see sounds or taste colors. I have long been familiar with such stories. But since the most extreme cases are the most widely reported, I had always assumed synesthesia was rare and typically severe. However, the condition is really a lot broader and not uncommon. Simply put, when synesthesia occurs, the stimulation of one brain area results in automatic, involuntary, and consistent stimulation of an unrelated brain area. You don’t need to actually see or hear this additional stimulation, as it may just occur in your mind’s eye. If a mention of the year 1992 reliably makes you visualize a location on an imaginary timeline with a distinct color and spatial position, then you are a synesthete. I picture years like this. The 90s are yellow and proceed toward the lower right, before the timeline makes a sharp curve up again around the turn of the century. I have comparable mental maps for months, numbers, the alphabet, even biological classification. So, my recent readings about synesthesia surprised me in several ways. First, that most people do not do this. Second, that it puts me in the same category as the guy who feels a smooth cold pole in his hand when he tastes mint. And third, that the few times when I have experienced mild altered sensory perception were not isolated incidences but part of a larger process.

Synesthesia is neither a talent nor a handicap. Although it’s an instructive window into brain function, it doesn’t give you any real advantage or disadvantage. It is, however, useful as a metaphor for, well, metaphor. A meta-metaphor, if you will. I’ve written before about the need to not take metaphors literally in science. Still, I’m fond of metaphors, and frequently apply them to explore complex concepts. Discussing synesthesia is similarly poetical. Both poets and synesthetes may describe someone as “blue”. For synesthetes, the brain is constantly dishing out metaphors whether we want them or not. But synesthetes are not confused about reality. No one thinks Wednesday is inherently indigo blue, in a sense that exists outside their heads. Synesthetes navigate the border between illusion and reality in a way that illuminates the role of metaphor in scientific understanding.

As I said, I occasionally experience synesthetic sensory stimulation. For example, if I am drifting off to sleep in a dark room, a sudden loud noise will produce a visible flash before my eyes. But the most striking case occurred when I was teaching high school many years ago. It was a small public school in New York City. The 10th grade was divided into four classes that traveled together as units. So I taught a single biology course four times a day to a different set of students, but always in the same room. Since the same classmates spent each day together, it didn’t take long for each class to develop its own distinct culture and behavior patterns. As I built relationships with the students, it got to the point where teaching each class felt like a very different experience. One day it suddenly occurred to me that the room was a different color for each class. It had been that way for weeks. It was a startling revelation. This was not a hallucination per se. It’s not like the walls actually turned color for me. Nor the students. I was well aware that there were many different colored objects in my field of vision. Just that, when class 10-4 was in there, it felt like I was in a mostly greenish space. Green things in the room stood out more and felt prominent. Then 10-4 left, and when 10-3 came in later that day, the same room seemed primarily yellow. But not the yellow of the 1990s.

I should emphasize that teaching 10th grade was the most challenging thing I have ever done. I suspect the illusion was stress-induced. But at its core it was the same thing as my mental pictures of years or numbers: an effective yet involuntary mnemonic. I needed to rapidly learn a hundred or so students, and so my mind assigned them color categories to facilitate the task. Just like we can use metaphor for summary and easy recall of information. But I had to pay attention. As with metaphor, the boundary where reality takes over isn’t always obvious.

Most of my experience with synesthesia is essentially visual thinking. Take taxonomy. Biologists often speak as if taxonomic categories are locations in physical space. As in, “A diverse array of pelt patterns occurs in mammals.” As a synesthete, I take the metaphor a step farther and actually picture a location called “mammals.” It’s not really a location. You can’t literally sit on a branch of the evolutionary tree. But you are literally a mammal. That’s just a simplified way of saying that you are a particular arrangement of atoms with a structure partially determined by hereditary information that has accumulated with modification over millions of years in one specific lineage of unbroken information transmission. “Mammal” is a region of abstract morphospace, not real space, but it is still a real biological entity. The distinction is subtle, and people sometimes assume a literal scientific fact is just a metaphor. For example, I often hear the well-meaning but false correction, “Humans didn’t evolve from monkeys, we just share a common ancestor with monkeys.” The truth is, by any reasonable definition, the common ancestor we shared with monkeys was itself a monkey. Old World monkeys and New World monkeys do not share a common ancestor any more recently than that same ancestor they share with us. So whatever combination of traits you want to use to define “monkey,” they must have been present in that ancestor. Your million-greats-grandmother wasn’t like a monkey, she was a monkey. If you want to get technical, so are you. Think spatially. Imagine evolutionary groups as a set of nested compartments. The large compartment of “animals” contains a smaller set called “mammals,” which contains a smaller set called “monkeys,” and eventually we get down to you.

My 10-2 students weren’t literally blue. They were literally monkeys, as are we all. But even if folks occasionally confuse metaphor with reality, the value of metaphors outweighs their cost. Similarly, rather than clouding my mind, my visual thought process helps me to parse complex concepts, scientific or otherwise. This is not to say that synesthetes make better scientists. In fact, one thing the book points out is that synesthesia is not fundamentally different from the connections across brain regions that happen in everyone. We can all appreciate poetry and metaphor. If we use them wisely, they can even help us to grasp the unvarnished facts.