RENEE MONTAGNE, host:

This is Morning Edition from NPR News. I'm Renee Montagne. This morning in "Your Health," a cancer that's become increasingly common, but is not as deadly as it used to be. We'll get to that story in a few minutes. First we'll hear something new about how the brain handles memories. In the early stages of Alzheimer's disease, people often remember events from childhood with perfect clarity, but they may have trouble recalling what happened yesterday. A new study explains why this happens. NPR's Jon Hamilton reports.

JON HAMILTON: Recent memories depend on a structure deep in the brain called the hippocampus. Scientists know that from studying people like Clive Wearing, a British musician. More than two decades ago, Wearing's life changed forever when a brain infection caused severe damage to his hippocampus. In this scene from a BBC documentary, Wearing and his wife, Deborah, are having a pretty typical conversation.

(Soundbite of BBC documentary)

Ms. DEBORAH WEARING: I'm going to see your kids tomorrow.

Mr. CLIVE WEARING (British Musicologist; Conductor; Keyboardist): You're going to see my kids?

Ms. WEARING: Yeah, your children.

Mr. WEARING: What are they up to now?

Ms. WEARING: Do you know what they're up to now?

Mr. WEARING: No, no, no.

Ms. WEARING: Guess what you think they're up to.

Mr. WEARING: No idea. Couldn't guess.

HAMILTON: Even though his wife told him just a few minutes earlier. Studies of people like Clive Wearing have made it pretty clear that the hippocampus plays a critical role in storing new memories and retrieving recent ones. Larry Squire, a researcher at UC San Diego says that fits in with what's known about Alzheimer's.

Dr. LARRY SQUIRE (Professor of Psychiatry, Neurosciences, and Psychology, University of California, San Diego): The reason that Alzheimer's disease begins with memory problems typically is because the areas that are damaged early in the disease are exactly these areas that we've been speaking about.

HAMILTON: But it's been less clear why older memories stay intact for longer. So Squire's scanned the brains of 15 healthy people in their 50s and 60s. He watched their brains as they tried to answer questions about news events from the past 30 years. Questions like...

Dr. SQUIRE: Where did a large airline accident take place involving two 747s? Things like that, where people might only be correct 60 or 70 percent of the time.

HAMILTON: That airline disaster took place in the Canary Islands in 1977. But Squire says the study wasn't about getting the right answers, it was about seeing which parts of the brain became active when people tried to retrieve memories from different time periods.

Dr. SQUIRE: And what we found was that the hippocampus was most active when subjects were recalling memories about news events that occurred just a year or two earlier. And the hippocampus became less active as subjects recalled memories that were five years and 10 years old.

HAMILTON: And even less active for stuff that happened before that. So, if the hippocampus wasn't involved in retrieving these old memories, what part of the brain was? The study offers some clues. It found that old memories trigger a lot of activity in the cerebral cortex, the surface layer of the brain. Squire says this observation could help explain one of the brain's most impressive feats.

Dr. SQUIRE: Events can be learned in an instant and still last for a lifetime.

HAMILTON: Because the brain gradually archives important information into protected storage sites. The new research published in The Journal of Neuroscience is generating a lot of talk among scientists.

Dr. RUSSELL A. POLDRACK (Professor of Psychology, UCLA): It's pretty compelling evidence.

HAMILTON: Russell Poldrack is a professor of psychology at UCLA.

Dr. POLDRACK: It's the clearest study to demonstrate, you know, what happens to memories as they get older.

HAMILTON: Poldrack says Squire's work seems to confirm not only work involving patients with brain damage, but also an observation about dementia made more than 200 years ago.

Dr. POLDRACK: The idea that older memories are more likely to survive actually is called Ribot's law. And Ribot was a 19th century scientist in Europe.

HAMILTON: Back then, most people didn't live long enough to get Alzheimer's disease. Now that they do, there's a compelling reason to understand why some memories persist and others slip away. Jon Hamilton, NPR News. Transcript provided by NPR, Copyright NPR.