Brain cells which encode spatial information form “geotags” for specific memories are activated immediately before those memories are recalled, a team of neuroscientists from the University of Pennsylvania and Freiburg University has found.
This 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 psychology professor Michael Kahana, of Penn’s School of Arts and Sciences.
Brain Activity Capture
Kahana and his colleagues have been conducting 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.
This spatial memory 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 to remember and name as many of the items they had delivered in whatever order they came to mind.
Episodic Memory, Spatial Association and Virtual Reality
In this way, the researchers were able to associate the neural activation linked 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.”
Researchers were able to build up a neural map that corresponded to the city’s layout, using the brain recordings generated while the participants navigated the city.
As participants passed by a specific 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.
Cross Referencing Memories
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.