A native Californian, Jackson went east to Massachusetts for college—that’s where he met Linscott. From there, he moved to Princeton to study the cognitive science of religion, examining the ways the human mind acquires and shares religious belief. “I wanted to understand how other people thought,” he says. “It felt like I had spent my life translating in one direction.” This was before his diagnosis, when he was aware of a yawning gap between his thoughts and other people’s but could not explain it.

Trying to bridge that gap could be frustrating. Jackson seemed to do well at Princeton, but underneath a gregarious façade, he was struggling. At his lowest point, he was spending up to twenty hours a day in the library poring over cognitive science research, in search of an explanation for what made his, and all brains, work. “By then it had become difficult to interact,” he says. “It exhausted a lot of people.”

Sometimes, walking home after a long study session, Jackson’s visual field was so cluttered with ideas that he had to sit down, call his girlfriend, and ask for a ride.

“The best metaphor I can give: When you were a kid did you ever just stare at a flashlight and turn out the light in the dark, and the light seems like it’s there flashing in front of your eyes?” he says. “That’s what it’s like, only with high-dimension geometry.”

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Jackson’s difficulties reflect a larger issue in synesthesia research: the difficulty of translating, or even attempting to explain, individual perception. Trying to understand how someone else sees the world can be like picturing an animal you’ve never seen based only on that person’s description. And that, unsurprisingly, can lead to misunderstanding.

For researchers, tackling this requires asking questions about biology and the building blocks of perceptual experience. Does synesthesia stem from something as fundamental as a sound wave hitting an eardrum, for instance? Or does it arise further along the thought assembly line, where our understanding of sensations is processed and expressed? This distinction can be traced to a foundational 1929 study that asked subjects to name and ascribe traits to two shapes—one spiky and jagged like an uneven star, the other bulbous and blobby like a mutated flower. The study, and a follow-up performed in 2001, found that, given two name options, speakers across languages and cultures chose “baluba” in the earlier study and “bouba” in the later study for the curvy blob, and “takete” in the earlier study and “kiki” in the later study for the spiky star. And not only that, they also tended to agree on which shape would be nervous, which laidback; which high-class, which low-class; and a number of other traits.

Today, nearly 90 years later, Danko Nikolic, a researcher at the Max Planck Institute in Germany, draws from “bouba and kiki” to argue that it is the ideas synesthetes use to categorize sensory information that prompt synesthesetic experiences. Our ideas and our senses are inextricably linked, he says, and that means (at least in the case of synesthesia) that one can’t exist without the other. He points to one study, in which synesthetes who see numbers or letters in specific colors were shown a shape that could be interpreted as an “S” or a “5” in a field of other symbols. When those symbols were numbers, most of them saw the shape as a “5”; when the symbols were letters, that interpretation changed to an “S”—and the corresponding color changed with it.