Chuck Bednar for redOrbit.com – @BednarChuck
What makes people recall some memories in such detail that it is almost as if they have traveled back in time and are reliving the events being described? A new study from a team of Vanderbilt University researchers sheds light on the phenomenon known as “mental time travel.”
As an example, the researchers refer to Proust’s novel Recollection of Things Past, in which the narrator begins an in-depth tale of events from his past after being triggered by the unique smell of a lemon madeleine. The resulting story is so detailed that it fills seven chapters.
In the new paper, published Wednesday in the Journal of Neuroscience, assistant psychology professor Sean Polyn and his colleagues set out to learn more about the mechanics that occur in the brain during such recall events. They did so by analyzing the brain activity of individuals as they performed a simple task involving memory recollection.
They found that the activity patterns in one specific region of the brain could be used to better predict the order in which people recalled information that they had only recently studied.
“It’s extremely important that we understand what different brain regions are doing as we search through our memories,” Polyn said. “Diseases like Alzheimer’s and epilepsy are devastating to memory, and this information can help us develop treatments to preserve patients’ memories, and identify adverse effects that new psychotropic drugs may have on people’s memory.”
Experts have long known that the brain’s medial temporal lobe (MTL) plays an important part in memory, because damage to this area can cause amnesia or other memory issues. However, they had not previously been able to determine how the mind governs how faithfully and accurately it can reproduce an individual memory.
Of course, as the researchers point out, not all memories are the same. High-fidelity memories such as the ones features in Proust’s novel are one extreme, and at the other end of the spectrum are the bits and pieces of memories that a person can recall clearly, but without supporting detail.
Memory model
Working alongside doctoral students James Kragel and Neal Morton, Polyn came up with a new model that can account for how the structures of the MTL support memory retrieval. Their model revealed that the anterior region of the lobe signals when a memory is being retrieved, but does not indicate the level of detail. The posterior region of the MTL, however, becomes active only when a person is experiencing a detailed, “time travel” type of memory.
In order to test their model, the Vanderbilt researchers recruited seven male and 13 female subjects between the ages of 18 and 35. Each individual was placed in an fMRI brain scanner and given a list of 24 names of common objects like horse, boat, and window.
They were then asked to decide if each object was large or small, as well as animate or inanimate, as a way to force them to focus on the words. Finally, following a short pause, the participants were asked to recall the words they had just studied in the correct order.
“The researchers found that when a participant’s brain scan indicated that they recalled an object with high fidelity, then their next response was likely to be the next item on the list,” the university said. “However, when the brain scan indicated that the object was an isolated recollection, then the next object the person recalled could come from anywhere on the list.”
Time travel to remember
For instance, if a participant was asked to study the words “horse, window, robot, and boat” in that order, he or she would be said to have experienced a “time travel” memory if recalling the word “horse” meant that they were able to immediately remember that “window” came right afterwards. On the other hand, if the person’s memory of “horse” was low fidelity, his or her next response would be the more distant word “boat,” the researchers explained.
“This model was much better than chance at predicting what a person would recall next,” Polyn said, “but when we told the model what a person’s brain activity was at the moment they recalled a particular item, the model became much better at predicting which item would be recalled next. This demonstrates that the brain stamps memories with a temporal code.”
“These time-travel recollections allow the brain to retrieve that temporal code, which makes memories for nearby things more accessible, in this case the next item in the list,” he added, explaining that the temporal code is similar to the time stamp placed on computer files.
When a person searched for files marked with a specific date and time, they search through all of the ones saved with at the time specified, the researchers explained. The process, in which time-travel memories are recalled, dubbed “reinstatement” by psychologists, is similar. However, it is also more flexible, as memories not just from the exact date and time but also those from around the same time can also be recollected.
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