Think back to an important event in your life: a graduation, a birth, a special Thanksgiving dinner. Chances are you're remembering not only what happened, but also where it happened. And now scientists think they know why.
As we form so-called episodic memories, the brain appears to be using special cells in the hippocampus to "geotag" each event, researchers report in Science. The process is similar to what some digital cameras do when they tag each picture with information about where the image was taken.
As a result of this automatic geotagging, memories about places and events are "fused together," says Michael Kahana, a psychologist at the University of Pennsylvania, and one of the study's authors. "You come to a location where something happened and it reminds you of an event," he says. "Or you think of an event and it reminds you of the place where it happened."
Kahana was part of an international team of scientists that figured out how the brain's geotagging system works by studying seven epileptic patients in Germany. The patients were awaiting surgery and had wires in their brains that allowed the researchers to measure the activity of individual brain cells. That gave Kahana's team a way to watch what happened as memories were formed and retrieved.
Patients in the study played a video game involving a virtual town. In the early stages of the game, "you drive around the town and you learn where the different locations are that matter," Kahana says. Once the players had formed a mental map of the town, the game had them drive to specific locations, like the toy store or the flower shop or the bakery.
Meanwhile, the researchers were monitoring activity in each player's hippocampus, which is where the brain creates the mental maps that help us navigate. These maps rely on special "place" cells that become active when we reach a specific location. And the researchers were able to identify place cells in each player that responded to specific locations in the virtual town.
Once these cells were identified, the players began the last part of the video game. Now each time they arrived at a location, they learned what item they had delivered there. "So when you get to the bakery, a voice will come on and tell you you've just delivered a zucchini," says Kahana.
This created mental links between places (like the bakery) and events (like delivering a zucchini). Then came the hard part: finding evidence of those links in the brain.
To do this, the researchers monitored the activity of place cells while players recalled specific objects they had delivered. And the team found that just before a person remembered they had delivered a zucchini, there was a burst of activity in the cells associated with the bakery — the place where the delivery was made.
This suggests the hippocampus is constantly using place cells to geotag events in our lives, Kahana says.
The finding reveals a lot about how the brain provides context to episodic memories, says Howard Eichenbaum, of Boston University. Knowing where something happened is one important bit of context. But it's also important to know when something happened, he says.
Cells that act as time stamps haven't been found yet in a human brain, Eichenbaum says. But he has found cells that perform this function in the rat hippocampus.
"So it seems like the hippocampus maps things in time," Eichenbaum says, "very much the way it maps in space."
ARI SHAPIRO, BYLINE: It's ALL THINGS CONSIDERED from NPR News. I'm Ari Shapiro.
Think back to an important event in your life: a wedding, a graduation, maybe a special Thanksgiving dinner. Chances are you remember not only what happened, but where it happened. For years, scientists have tried to figure out how the brain creates this link between an event and a place.
NPR's Jon Hamilton reports on a new study that seems to have an answer.
JON HAMILTON, BYLINE: Michael Kahana, from the University of Pennsylvania, says what's remarkable about the links between places and events is how strong they are.
MICHAEL KAHANA: You come to a location where something happened and it reminds you of the event. Or you think of an event and it reminds you of the place where the event happened.
HAMILTON: Just ask a Red Sox fan where they were when their team won the World Series a few weeks ago.
Kahana wanted to know how the brain forges these powerful links. So he teamed up with researchers in Germany to study the brains of seven patients with severe epilepsy.
KAHANA: These are patients who are undergoing a neurosurgical effort to map seizures in the brain so they have electrodes implanted.
HAMILTON: Which allowed the researchers to see what individual brain cells were doing, as patients formed new memories and then retrieved them. The team focused on cells in the hippocampus, an area that creates the mental maps we use to navigate in the world. Kahana says the patients played a video game involving a virtual town.
KAHANA: And the role that you play in this virtual town is you drive around the town and you learn where the different locations are that matter.
HAMILTON: Once the players had formed a mental map of the town, Kahana says, they were asked to drive to specific locations, like the toy store or the bakery.
KAHANA: And each time they arrive at one of these locations, they will be informed that something happened. And the something that happened is they delivered an object. So when you get to the bakery, a voice will come on and tell you you've just delivered a zucchini.
HAMILTON: That created mental links between places, like the bakery, and events, like delivering a zucchini. After the game ended, the scientists asked players to remember the objects they had delivered. And Kahana says activity in the hippocampus suggested how the brain was linking places and events.
KAHANA: The brain is doing a kind of automatic geotagging.
HAMILTON: He says it's doing this with special cells that start firing when we reach a specific location, like the bakery. And the experiment showed that these same cells also start firing when people recall something that happened at that location, like delivering a zucchini. Kahana says the cells appear to be adding information about location to our memory of an event.
KAHANA: When you're trying to remember the event, that geotag pops up. And by popping up what I mean is simply that those neurons in the hippocampus that told you where you were, those neurons reactivate just before you remember zucchini.
HAMILTON: That suggests the neurons associated with a place actually help us retrieve memories about what happened there.
Howard Eichenbaum, of Boston University, says the new study reveals a lot about how the hippocampus provides context to our memories of events. But he says location isn't the only sort of context that matters. It's also important to know when something happened. And Eichenbaum says his research turned up another set of specialized cells in the rat hippocampus that put a sort of time stamp on memories.
HOWARD EICHENBAUM: So it seems like the hippocampus maps things in time exactly the way it maps in space. In fact, it's arguably just another dimension of our experience.
HAMILTON: Eichenbaum says having this time stamp on each memory lets us do something very important.
EICHENBAUM: It allows us to replay events in our heads in the order in which they happened.
HAMILTON: Eichenbaum says that ability, when combined with location information, can be a lifesaver. Say you're an animal that recently survived an encounter with a predator. Eichenbaum says you might avoid a second encounter if you can remember where the first one occurred and what sequence of events led to your brush with death.
EICHENBAUM: For survival the key feature would be what was I doing just before I got myself into this mess.
HAMILTON: The new research appears in the journal Science.
Jon Hamilton, NPR News.
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