GWEN IFILL: Finally, the extraordinary tale of one of the world's most famous brains. That's the focus of tonight's premiere of "Nova scienceNOW." A warning: There are explicit medical images in the excerpts we're about to show you.
NARRATOR: This is the brain that more than any other in history has allowed scientists to make sense of your brain. For 82 years, it resided in the head of a man named Henry Gustav Molaison, better known as H.M. He was perhaps the most studied patient ever, and that didn't end when he died last year.
With H.M.'s permission, neuroanatomist Jacopo Annese went to work dissecting his brain into 3,000 slices.
JACOPO ANNESE, The Brain Observatory, UCSD: You can imagine the brain being like a book, and our tissue slices are the pages of this book. The only catch is, the slices are transparent, so you cannot really see anything, until you use a lot of obscure chemical processes to reveal the features in the tissue.
Eventually, the entire book will be completely stained, and it will tell us the story of this brain.
NARRATOR: It's a story that begins with epilepsy, epilepsy so severe that, by 1953, H.M. had reached his breaking point.
SUZANNE CORKIN, Brain and Cognitive Sciences, MIT: He had to stop working because of the frequency of his seizures. It was just too dangerous. So he was basically at home with his parents; his life was on hold.
NARRATOR: In desperation, H.M. let Surgeon William Scoville remove slivers of brain on either side of his head, each containing a seahorse-shaped structure called a hippocampus. This might have seemed reasonable at a time when we knew almost nothing about memory, and it did quiet his seizures, though at a terrible cost.
DOCTOR: Do you know what you did yesterday?
HENRY GUSTAV MOLAISON: No, I don't.
DOCTOR: How about this morning?
HENRY GUSTAV MOLAISON: That I don't know myself. I can't tell you, because I don't remember.
NARRATOR: H.M.'s condition might have seemed like simple dementia. But as neuroscientist Brenda Milner discovered, it was anything but.
BRENDA MILNER, Montreal Neurological Institute: He would say, "Right now it's like waking from a dream. Right now everything is clear. But what happened just before?"
NARRATOR: Milner found that H.M. had a normal I.Q. He could crack jokes, solve puzzles, even to some extent remember.
ERIC KANDEL, Columbia University: So H.M. could remember everything that happened prior to the operation. He could remember the trauma of his childhood. He could remember going to elementary school, to high school, working in the assembly plant.
NARRATOR: What he couldn't do was hold on to new information for more than a few minutes. In a moment of insight, Milner concluded the hippocampus must make long-term memories out of short-term ones.
SUZANNE CORKIN: That was a groundbreaking finding, because it showed that the ability to establish long-term memories is localized to this tiny area in the brain.
NARRATOR: If H.M. had contributed nothing more, his fame would have been assured. But he would go on inspiring discoveries for decades. The next involved a pencil, a mirror, and again Brenda Milner.
JACOPO ANNESE: She did this brilliant test in which she had H.M. draw the outlines of a star, without looking at the star, but looking into a mirror.
NARRATOR: It's hard at first to draw within the line. But could H.M., like people with normal memory, learn to do it with practice? After 3 days and 10 trials, his performance was nearly perfect.
BRENDA MILNER: "Well," he said, "I thought this would be difficult, but it looks as though I've done pretty well." He had no memory of all these learning trials that he'd been through and the beautiful learning. That was the real contrast.
NARRATOR: So H.M. could remember a motor skill, but not recall a fact or an event. It was a key discovery, because it showed there were different kinds of memory dependent on different parts of the brain.
GWEN IFILL: "Nova scienceNOW" can be seen on most PBS stations tonight. Check your local listings for the time. You can also find a link to their Web site by going to ours, newshour.pbs.org.