Here are five patients, from Kean's book, whose stories transformed neuroscience:

The Man Who Could Not Imagine the Future

Kent Cochrane (KC), pictured below, was a '70s wild child, playing in a rock band, getting into bar fights, and zooming around Toronto on his motorcycle. But in 1981, a motorcycle accident left him without two critical brain structures. Both of his hippocampi, the parts of the brain that allow us to form new long-term memories for facts and events in our lives, were lost. That's quite different from other amnesiacs, whose damage is either restricted to only one brain hemisphere, or includes large portions of regions outside of the hippocampus.

KC's case was similar to that of Henry Molaison, another famous amnesiac known as HM. HM taught us that conscious memories of things like which street you grew up on (personal semantic information or facts about yourself) and what happened on your prom night (episodic memories for events in your past) are stored independently from other types of nonconscious memories, of things like how to ride a bike or play the guitar. You can lose one type of memory without losing the other. But KC taught us still more: That our ability to imagine the future is tied to our ability to use our memories to reexperience the past.

"When he lost his past self," says Kean of KC, "he lost all sense of what he was going to do over the next hour, or over the next day, or over the next year. He couldn't project himself forward at all, and kind of realize that he would want to be doing something in a month or a year. He was kind of eternally trapped in the present tense."

Although it might sound obvious now, before KC came along, neuroscientists hadn't realized how closely tied, on a cognitive level, our future is to our past. "But if you think about it, it does make sense," explains Kean, "because the ultimate biological purpose of having a memory isn't just…to make you happy or something like that. The point of a memory is so that you can kind of keep track of what happened in your past, and then apply that to the future."

The Man Whose Vocabulary Was Reduced to One Word

In the late 18th century, the idea that different functions of the mind might be tied to specific parts of the brain first gained a foothold. Phrenology, as it came to be called, was based on the notion that bumps in the skull were markers of larger bits of brain, and that these bumps were clues as to what mental talents, or lack thereof, a person might possess. By the 1840s, however, many scientists dismissed phrenology (and rightly so) as rank pseudoscience.

So when Paul Broca, a French neuroanatomist, first proposed that there was a specific "language area" in the brain—and did so based on evidence from the brain of a patient nicknamed "Tan"—he was laughed out of a scientific meeting.

Wikimedia Commons

Tan—whose story is related in Kean's new book—suffered from epilepsy throughout his childhood. By age 31, he could only respond to questions by repeating the word "tan." Unless, that is, he was enraged. Then, he'd let out a cry of "Sacre nom de Dieu!" a French insult. Yet Tan still seemed to be able to understand spoken language, even if he could not to speak himself. Because his vocabulary was so impoverished, he became an expert at gesturing, expressing himself through mime.