Researchers found the brain "geotags" memories with information about where they were formed.
The team asked study participants to deliver items to certain locations in a digital town, and observed how "brain cells that encode spatial information form 'geotags' for specific memories and are activated immediately before those memories are recalled," a University of Pennsylvania news release reported.
This phenomenon could explain why a certain memory can bring up recollection of other things 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," Michael Kahana, professor of psychology in Penn's School of Arts and Sciences, said.
The participants were first allowed to freely explore the virtual town. After they had gotten familiar with the landscape they were asked to make deliveries to different stores. Once they made the delivery they were informed of what the item had been and given instructions for their next task.
After the participants made 13 deliveries the screen went black and they were asked to recall as many deliveries as they could remember in the order they "came to mind."
"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 the participants what item they delivered instead of what locations they had visited the researchers were able to see if spatial memory systems were closely linked with episodic memories.
"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."
The team recorded the participants' brain activity during the trial, and were able to make a neural map that was accurate with the virtual city's layout. They noticed the neurons responsible for a certain part of the map activated right before the participant recalled the item that had been delivered to that location.
"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."
The finding supports the theory that the hippocampus not only tracks the location of special memory, but records it for later episodic memory as well.
"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.
