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Researchers also learned more about another kind of remembering from HM. Even though he couldn't say what he'd had for breakfast or name the current President, there were some things that he could remember. Milner found that he was capable of learning complicated tasks without even realizing it. In one study, she showed that HM could learn how to trace inside a five-pointed star on a piece of paper while looking at its reflection in a mirror. Each time Milner gave HM the task, he claimed never to have tried it before. And yet, each day his brain got better at guiding his hand to work in reverse. Despite his amnesia, he was remembering.

Though there is disagreement about just how many memory systems there are, scientists generally divide memories into two types: declarative and nondeclarative (sometimes referred to as explicit and implicit). Declarative memories are things you know you remember, like the color of your car or what happened yesterday afternoon. EP and HM have lost the ability to make new declarative memories. Nondeclarative memories are the things you know without consciously thinking about them, like how to ride a bike or how to draw a shape while looking at it in a mirror. Those unconscious memories don't rely on the hippocampal region to be consolidated and stored. They happen in completely different parts of the brain. Motor skill learning takes place at the base of the brain in the cerebellum, perceptual learning in the neocortex, habit learning at the brain's center. As EP and HM so strikingly demonstrate, you can damage one part of the brain, and the rest will keep on working.

The metaphors we most often use to describe memory—the photograph, the tape recorder, the mirror, the hard drive—all suggest mechanical accuracy, as if the mind were some sort of meticulous transcriber of our experiences. And for a long time it was a commonly held view that our brains function as perfect recorders—that a lifetime of memories are socked away somewhere in the cerebral attic, and if they can't be found it isn't because they've disappeared, but only because we've lost access to them.

A Canadian neurosurgeon named Wilder Penfield thought he'd proved that theory by the 1940s after using electrical probes to stimulate the brains of epileptic patients while they were lying conscious on the operating table. He was trying to pinpoint the source of their epilepsy, but he found that when his probe touched certain parts of the temporal lobe, the patients started describing vivid experiences. When he touched the same spot again, he often elicited the same descriptions. Penfield came to believe that the brain records everything to which it pays any degree of conscious attention, and that this recording is permanent.

Most scientists now agree that the strange recollections triggered by Penfield were closer to fantasies or hallucinations than to memories, but the sudden reappearance of long-lost episodes from one's past is an experience surely familiar to everyone. Still, as a recorder, the brain does a notoriously wretched job. Tragedies and humiliations seem to be etched most sharply, often with the most unbearable exactitude, while those memories we think we really need—the name of the acquaintance, the time of the appointment, the location of the car keys—have a habit of evaporating.

Michael Anderson, a memory researcher at the University of Oregon in Eugene, has tried to estimate the cost of all that evaporation. According to a decade's worth of "forgetting diaries" kept by his undergraduate students (the amount of time it takes to find the car keys, for example), Anderson calculates that people squander more than a month of every year just compensating for things they've forgotten.

AJ remembers when she first realized that her memory was not the same as everyone else's. She was in the seventh grade, studying for finals. "I was not happy because I hated school," she says. Her mother was helping her with her homework, but her mind had wandered elsewhere. "I started thinking about the year before, when I was in sixth grade and how I loved sixth grade. But then I started realizing that I was remembering the exact date, exactly what I was doing a year ago that day." At first she didn't think much of it. But a few weeks later, playing with a friend, she remembered that they had also spent the day together exactly one year earlier.

"Each year has a certain feeling, and then each time of year has a certain feeling. The spring of 1981 feels completely different from the winter of 1981," she says. Dates for AJ are like the petite madeleine cake that sent Marcel Proust's mind hurtling back in time in Remembrance of Things Past. Their mere mention starts her reminiscing involuntarily. "You know when you smell something, it brings you back? I'm like ten levels deeper and more intense than that."

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