A still from the game participants played. They made deliveries to stores, then were asked to recall what they had delivered. (Credit: University of Pennsylvania)
New research has confirmed that cells in the hippocampus responsible for processing memories encodes ("geotags") the memories with spatial information that allows us to remember where and when it occurred. Recalling that memory can trigger other memories from the same location.
Science Daily offered a god summary of the press release, and NPR spoke with one of the principle researchers involved in the study.
Nov. 28, 2013 — Using a video game in which people navigate through a virtual town delivering objects to specific locations, a team of neuroscientists from the University of Pennsylvania and Freiburg University has discovered how brain cells that encode spatial information form "geotags" for specific memories and are activated immediately before those memories are recalled.
Their work shows how spatial information is incorporated into memories and why remembering an experience can quickly bring to mind other events that happened in the same place.
"These findings provide the first direct neural evidence for the idea that the human memory system tags memories with information about where and when they were formed and that the act of recall involves the reinstatement of these tags," said Michael Kahana, professor of psychology in Penn's School of Arts and Sciences.
The study was led by Kahana and professor Andreas Schulze-Bonhage of Freiberg. Jonathan F. Miller, Alec Solway, Max Merkow and Sean M. Polyn, all members of Kahana's lab, and Markus Neufang, Armin Brandt, Michael Trippel, Irina Mader and Stefan Hefft, all members of Schulze-Bonhage's lab, contributed to the study. They also collaborated with Drexel University's Joshua Jacobs.
Their study was published in the journal Science.
Kahana and his colleagues have long conducted research with epilepsy patients who have electrodes implanted in their brains as part of their treatment. The electrodes directly capture electrical activity from throughout the brain while the patients participate in experiments from their hospital beds.
As with earlier spatial memory experiments conducted by Kahana's group, this study involved playing a simple video game on a bedside computer. The game in this experiment involved making deliveries to stores in a virtual city. The participants were first given a period where they were allowed to freely explore the city and learn the stores' locations. When the game began, participants were only instructed where their next stop was, without being told what they were delivering. After they reached their destination, the game would reveal the item that had been delivered, and then give the participant their next stop.
After 13 deliveries, the screen went blank and participants were asked to remember and name as many of the items they had delivered in the order they came to mind.
This allowed the researchers to correlate the neural activation associated with the formation of spatial memories (the locations of the stores) and the recall of episodic memories: (the list of items that had been delivered).
"A challenge in studying memory in naturalistic settings is that we cannot create a realistic experience where the experimenter retains control over and can measure every aspect of what the participant does and sees. Virtual reality solves that problem," Kahana said. "Having these patients play our games allows us to record every action they take in the game and to measure the responses of neurons both during spatial navigation and then later during verbal recall."
By asking participants to recall the items they delivered instead of the stores they visited, the researchers could test whether their spatial memory systems were being activated even when episodic memories were being accessed. The map-like nature of the neurons associated with spatial memory made this comparison possible.
"During navigation, neurons in the hippocampus and neighboring regions can often represent the patient's virtual location within the town, kind of like a brain GPS device," Kahana said. "These so-called 'place cells' are perhaps the most striking example of a neuron that encodes an abstract cognitive representation."
Using the brain recordings generated while the participants navigated the city, the researchers were able to develop a neural map that corresponded to the city's layout. As participants passed by a particular store, the researchers correlated their spatial memory of that location with the pattern of place cell activation recorded. To avoid confounding the episodic memories of the items delivered with the spatial memory of a store's location, the researchers excluded trips that were directly to or from that store when placing it on the neural map.
With maps of place cell activations in hand, the researchers were able to cross- reference each participant's spatial memories as they accessed their episodic memories of the delivered items. The researchers found that the neurons associated with a particular region of the map activated immediately before a participant named the item that was delivered to a store in that region.
"This means that if we were given just the place cell activations of a participant," Kahana said, "we could predict, with better than chance accuracy, the item he or she was recalling. And while we cannot distinguish whether these spatial memories are actually helping the participants access their episodic memories or are just coming along for the ride, we're seeing that this place cell activation plays a role in the memory retrieval processes."
Earlier neuroscience research in both human and animal cognition had suggested the hippocampus has two distinct roles: the role of cartographer, tracking
location information for spatial memory, and the role of scribe, recording events for episodic memory. This experiment provides further evidence that these roles are intertwined.
"Our finding that spontaneous recall of a memory activates its neural geotag suggests that spatial and episodic memory functions of the hippocampus are intimately related and may reflect a common functional architecture," Kahana said.
J. F. Miller, M. Neufang, A. Solway, A. Brandt, M. Trippel, I. Mader, S. Hefft, M. Merkow, S. M. Polyn, J. Jacobs, M. J. Kahana, A. Schulze-Bonhage. (2013, ). Neural Activity in Human Hippocampal Formation Reveals the Spatial Context of Retrieved Memories. Science; 342(6162): 1111-1114. DOI: 10.1126/science.1244056
From the original article:
In many species, spatial navigation is supported by a network of place cells that exhibit increased firing whenever an animal is in a certain region of an environment. Does this neural representation of location form part of the spatiotemporal context into which episodic memories are encoded? We recorded medial temporal lobe neuronal activity as epilepsy patients performed a hybrid spatial and episodic memory task. We identified place-responsive cells active during virtual navigation and then asked whether the same cells activated during the subsequent recall of navigation-related memories without actual navigation. Place-responsive cell activity was reinstated during episodic memory retrieval. Neuronal firing during the retrieval of each memory was similar to the activity that represented the locations in the environment where the memory was initially encoded.
Remembrance of Places Past
The hippocampus has two major roles in cognition. Place-responsive neurons form a context-sensitive cognitive map, firing more strongly when an animal traverses specific regions of its environment. Both humans and animals thus need the hippocampus to learn their way around novel environments. Similarly, the hippocampus is critical for our ability to remember a specific event in space and time. It has thus been suggested that the spatial and memory functions of the hippocampus reflect a common architecture. Recording from neurosurgical patients playing a virtual reality memory game, Miller et al. (p. 1111) found that the recall of events was indeed associated with reinstatement of the place-firing of neurons activated as the subjects navigated through the environment.
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by Jon Hamilton
November 28, 2013
Listen to the Story
4 min 15 sec
Benjamin Arthur for NPR
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.
Related Story: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."
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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 HAMIILTON, 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.
HAMIILTON: 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.
HAMIILTON: 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.
HAMIILTON: 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.
HAMIILTON: 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.
HAMIILTON: 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.
HAMIILTON: 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.
HAMIILTON: 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.
HAMIILTON: 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.
HAMIILTON: The new research appears in the journal Science.
Jon Hamilton, NPR News.