Past neuroscience and psychology studies have shown that after the human brain encodes specific events or information, it can periodically reactivate them to facilitate their retention, via a process known as memory consolidation. The reactivation of memories has been specifically studied in the context of sleep or rest, with findings suggesting that during periods of inactivity, the brain reactivates specific memories, allowing people to remember them in the long term.

Researchers at the University of Pennsylvania and other institutions in the United States recently conducted a study exploring the possibility that the brain engages in a similar reactivation process during wakefulness to store important information for shorter periods of time. Their findings, published in Nature Neuroscience, suggest that the spontaneous reactivation of specific stimuli in the brain during the brief intervals between their encoding predicts the accuracy with which people remember them at the end of a memory task.

“Mike Kahana and I were both quite interested in the long history of thinking about rehearsal and its effects on the way in which people later recalled things,” Dr. David Halpern, the first author of the paper, told Medical Xpress. “Rehearsal is challenging to study since people often do it without any overt behavior (unless we ask them to rehearse out loud).”

Dr. Michael J. Kahana’s lab at the University of Pennsylvania has been conducting studies related to memory processes for some time. This led to the creation of a dataset that includes memory task performance and matching intracranial electroencephalography (EEG) recordings from several people, offering insight into what happens in the brain while people attempt to remember information for short periods of time.

“We decided to try to look at when neural activity related to specific words seemed to be reactivated while they were looking at other words and see if that looked like what theorists in psychology thought was going on,” said Dr. Halpern. “In addition, there is this parallel literature on consolidation in neuroscience that also suggested that reactivation was key to understanding what gets remembered later.”

Many previous studies and theoretical psychology models suggest that memory consolidation processes primarily take place during sleep and over extended periods of rest. During these periods, the brain is not actively engaged in everyday activities and external perceptions, but rather focused on internal thoughts or processes.

“In addition, past works suggest that certain neurotransmitters are involved in long timescales, much longer than the time between two-word presentations,” said Dr. Halpern. “This meant that looking at neural reinstatement in our data would also have interesting implications for those theories as well.”

As part of their study, the researchers analyzed the data collected by the Kahana Lab, which measured neural activity using intracranial EEG. This is a technique that relies on implanted electrodes, which some patients diagnosed with epilepsy are already exposed to as part of their treatment (i.e., to identify the brain tissue that should be surgically removed to diminish or entirely stop their epileptic seizures).

“These patients need to wait in the hospital until they get enough measurements to determine the source, so a network of our research assistants across the country asked them if they would be willing to do a memory task while their neural activity is being recorded,” explained Dr. Halpern.

“The task we usually use is called free recall: we ask the patients to look at a sequence of words on a screen and then, after a short delay, recall as many of them as they can in any order.”

Dr. Halpern, Dr. Kahana and their collaborators combined intracranial EEG recordings collected while patients were completing a short-term memory task with their performance on the task (i.e., what they remembered and the order in which they remembered stimuli). The study participants’ ability to recall information and the order in which they remember it offers valuable insight into how memories are organized in their brains.

“Firstly, we found that there seems to be some process happening involving reinstatement of neural activity on a sub-second scale that is related to later memory performance,” said Dr. Halpern.

“This is a way in which the brain/mind can, after having an experience, make it more likely to be remembered later and perhaps alter the way in which it is remembered. It is interesting that the processes are, at first glance, quite similar to what we think is going on during sleep.”

Overall, the initial findings gathered by this team of researchers suggest that the brain also re-activates stimuli that we are trying to remember over short periods of time while we are awake. Further research could help to better understand the brain re-activation processes they observed and determine how they differ from those associated with memory consolidation during sleep or rest.

In the future, these results could also help devise interventions aimed at improving people’s ability to remember information in the short-term, which could be beneficial for both students and people diagnosed with memory disorders. Dr. Halpern has now left the Kahana Lab and now works in another research lab that is exploring the connections between memory and decision-making.

“I am very interested in the way in which rehearsal and reinstatement might be involved in the evaluation process that leads us to make a choice,” added Dr. Halpern. “Another question I am quite interested in exploring is, if we are routinely (perhaps subconsciously) rehearsing our past experiences, how does our brain keep these separate from our ongoing veridical perceptual experience?

“In theory, this should cause some interference in terms of remembering the order in which things happened, but this doesn’t seem to be the case, even in our data.”

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