Unravelling the brain’s inner GPS earned a trio of scientists the Nobel Prize in medicine last year, but of course that did not mark the end to this complex story. Scientists have now added another important piece to this puzzle, gathering evidence of a kind of episodic memory diary in which the time and location of specific memories are recorded.
Researchers discovered that a part of our brain’s learning and memory center – the hippocampus – seems to keep a log of where and when certain events occurred. Furthermore, they also found that the distance between these memories, both in terms of space and time, was reflected in their neuronal representations. In other words, the greater the distance between the events, the further apart they appeared in the hippocampus.
“What we’re picking up here is not the whole memory, but the basic gist – the where and when of the experience,” lead researcher Per Sederberg said in a statement. “This could be viewed as the memory hub, where we have these general, large-scale representations of our experiences.”
The study builds on the fairly recent discovery of so-called place and grid cells in rat hippocampi that create a map of the surrounding environment and bestow a sense of place. Since laboratory animals are confined to small areas and artificial situations, researchers wanted to expand on what we know by examining human memories from real-life events.
To do this, scientists from Ohio State University gathered a small group of nine women and dangled a smartphone from their necks for one month. During this period, a specially designed app took photos randomly throughout the day while also making a note of the time, location and whether the user was moving or stationary, among other things.
To study the memories formed during this time, images of brain activity were taken using an fMRI machine while the volunteers were presented with a random selection of their photos and asked to try and remember details of the experience shown. They then put the brain data into pairs, comparing activity from events that were a minimum of 100 meters (328 feet) and 16 hours apart. Since it’s difficult to identify what dimensions of experience represent the “total memory,” Sederberg explains to IFLScience, the researchers focused on the most objective measure possible with the data, which was when and where the episode was experienced.
As described in Proceedings of the National Academy of Sciences, they found that the similarity of activity patterns displayed in the anterior hippocampus while experiences were relived reflected how far apart the memories were in terms of space and time. So the more dissimilar the patterns, the further apart they were. But if participants couldn’t remember details of the event pictured, this relationship was not observed. Furthermore, they did not find this result when looking for spatial or temporal information independently; both had to be included to observe this effect.
“The biggest surprise was how entwined spatial and temporal information was in our data,” Sederberg tells IFLScience. “In hindsight this makes sense given that the foundation of an autobiographical memory is the time and place where the episode took place.”
According to Sederberg, the fact that the location of space and time information was in the anterior hippocampus provides some of the first direct support for an emerging theory that this region is critical for representing the gist of our experiences. And since this is one of the first areas to degenerate in Alzheimer’s, the findings could suggest that, at least in the early stages of dementia, individuals may still be able to form new memories, but are unable to retrieve them. This is something he hopes to explore further.
