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How Does the Brain Remember the Places of Your Past?

Scientists find that the hippocampus gives memories location-based "geotags."

A new study from the University of Pennsylvania has discovered how our memories are "geotagged" based on location using a GPS-like brain system. These "geotags" bring back a flood of memories associated with specific places from your past. The nostalgia of being home for the holidays is a perfect example of this type of memory encoding.

The hippocampus is responsible for our ability to remember a specific event in time and space. A team of neuroscientists from the University of Pennsylvania and Freiburg University recently discovered exactly how the brain encodes spatial information into "geotags" that are linked to specific memories. These “geotags” are activated immediately before a memory is recalled.

The new UPenn study titled, “Neural Activity in Human Hippocampal Formation Reveals the Spatial Context of Retrieved Memories” was published November 29, 2013 in the journal Science.

The hippocampus plays important roles in the consolidation of information from short-term memory to long-term memory and in spatial navigation. The researchers at UPenn had a hunch that spatial and memory functions of the hippocampus are a universal brain architecture. They were correct.

Using a group of neurosurgical patients, the researchers had test subjects play a virtual reality memory game. They found that the recall of events is directly linked to the geographical-firing of neurons activated as the subjects navigated through specific places in a virtual reality.

This research builds on previous findings from a March 2013 UCSF study which showed that when rats were navigating a maze, if the rat paused before deciding which way to turn, the hippocampus became more active and the animal was more likely to reach the target destination.

The Hippocampus

The UPenn researchers used a video game in which subjects navigate through a virtual town delivering objects to specific locations to identified place-responsive cells active during virtual navigation. The researchers discovered that the same cells are activated during the recall of navigation-related memories even without actual navigation taking place. The place-responsive “geotag" cell activity fired consistently during memory retrieval. Neuronal firing during the retrieval of each memory was coupled with the location in the environment where the memory was initially encoded.

This new research from UPenn shows how spatial information is incorporated into memories and why being back in a familiar location triggers positive and negative flashbacks of events that happened in that environment. The nostalgia of the holidays is a perfect example of this type of memory encoding. Both the joy and anxiety of the holidays are coupled with hippocampal 'geotags.' Being home for the holidays opens up these memory boxes and triggers a flood of old memories in vivid Technicolor like they happened yesterday.

For most of us, the locations we spend Thanksgiving and Christmas are coupled with strong memories and emotions linked to that environment. What locations do you associate with the holiday season? What specific memories do the geotags of being in familiar holiday locations stir up for you?

Michael Kahana, professor of psychology in Penn's School of Arts and Sciences, and lead author of the study says, "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.”

Spatial Memories are linked to Episodic Memories

As with earlier spatial memory experiments conducted by Kahana's group, their new study involved playing a simple video game on a bedside computer. The game in the UPenn 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 mixing up 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.

Once the researchers established the maps of place cell activations, they 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.

In 2011, a team of British researchers studied the hippocampus volume of London taxi cab drivers who need to gain "The Knowledge" of 25,000 streets and places of interest to qualify to be a city cab driver. The researchers examined the structure of the volunteers' brains at the start of the study, before any of the trainees had begun their training. They found no discernible differences in the structures of either the posterior hippocampus or the anterior hippocampus between the groups, and all groups performed equally well on the memory tasks.

Three to four years later—when the trainees had either passed the test or had failed to acquire "The Knowledge"—the researchers again looked at the brain structures of the volunteers and tested their performance on the memory tasks. This time, they found significant differences in the posterior hippocampus—those trainees that qualified as taxi drivers had a greater volume of grey matter in the region than they had before they started their training."

PTSD and Flashbacks May Be Linked to “Geotags”

These findings also help explain the brain science behind post-traumatic stress disorder (PTSD) and how something like being at a gas station or the smell of diesel are ‘geotagged’ and can trigger vivid flashbacks that take a veteran back to the battlefield.

A few years ago, I was jumped and beaten up by three guys a few blocks away from my apartment in the East Village of Manhattan. For months after the attack I avoided walking past the corner where I was mugged and the sidewalk that I had covered with blood... The geotags linked to the location where I was beaten up triggered physiological 'fight-or-flight' responses of increased heart rate, shortness of breath, sweaty palms etc.

I can walk past that patch of pavement now without having a full blown panic attack, but the hippocampal geotags linked to trauma that occured in that location will never totally go away. This type of ‘geotagging’ is probably a survival mechanism designed to hardwire a fear response that protects us from revisiting environments that aren’t safe and helps us to navigate the world without putting our lives in danger.

Conclusion: Increasing the Size of the Hippocampus Improves Memory

The good news is, you don't have to study a London street map or play video games in a virtual reaity to impove the function of your hippocampus. Multiple studies have found that physical activity and meditation can both bulk up the hippocampus and strengthen connectivity with other brain regions which will improve learning and memory at any age.

Researchers in 2010 found an association between physical fitness levels and the hippocampus size of 9 and 10-year-old children. The children who were more fit had a larger hippocampus and performed better on a test of memory than their less-fit peers.

In another study titled, "Effect of Physical Activity in Cognitive Function in Older Adults at Risk for Alzheimer's Disease," researchers found that elderly individuals who engaged in regular physical exercise for a 24-week period had an improvement of an 1,800 percent on measures of memory, language ability, attention, and other important cognitive functions compared to an age-matched group not involved in the exercise program. These improvements have been linked to increases in hippocampal volume.

Michael Kahana of UPenn concludes, "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.”

"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."

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