Research: Neuronal activity is needed to maintain short-term memory

Maintaining a thought in working memory—even if it's not the focus of our attention—requires sustained neural activity. This is demonstrated by recent research by Dr. Jan Kamiński's team at the Nencki Institute of the Polish Academy of Sciences, published in Nature Human Behavior.

Working memory – formerly called short-term memory – serves as our mental RAM or a whiteboard where we briefly record important information that we process before permanently storing it or erasing it. Working memory allows us to remember our BLIK number, the ingredients we've already added to a cake, or the digits in mathematical operations.

"We analyzed the neural mechanisms underlying working memory. Contrary to the popular activity-silent theory, we discovered that thoughts held in our working memory, which are not the focus of our attention, are still represented by active, sustained neural activity," explained study author Dr. Jan Kamiński from the Laboratory of Neurophysiology of the Mind at the Nencki Institute of the Polish Academy of Sciences in an interview with PAP.

- It seems that much of our thinking is based on the constant electrical activity of neurons - regardless of whether the thought is being used at the moment or just "waiting in the queue," adds Dr. Kamiński.

The team's research was published in the prestigious journal Nature Human Behavior. The paper's first author is Dr. Katarzyna Paluch.

Working memory allows us to store a lot of information simultaneously. An example? A memory game where the goal is to find pairs among face-down cards. When we reveal a card, we can recall the location of the other card that forms a pair with it. Information about this card briefly enters consciousness. At the same time, working memory—temporarily out of focus—contains ready-to-use information about the locations of other cards that have already appeared in the game.

The researchers' goal was to investigate how newly acquired information that is temporarily unused is stored. Previous research (primarily EEG and fMRI) suggested that inactive elements of working memory may be stored "silently," meaning without any apparent neural activity. The problem is that these techniques measure activity across large populations of neurons, which can mask subtle traces of activity in individual memory cells.

Dr. Jan Kamiński's team, however, took a novel approach: recording the activity of individual neurons in the human brain—a rarity in research. This was made possible by collaboration with patients suffering from drug-resistant epilepsy, in whom electrodes were implanted in the brain for diagnostic purposes.

"About 30% of epilepsy patients do not respond to pharmacological treatment. The most effective solution may be surgical removal of the seizure focus, but first it must be precisely located," explains Jan Kamiński.

As part of this diagnostic process, patients had electrodes implanted in the medial temporal lobe—an area crucial for memory. Using special microelectrodes, researchers were able to record the activity of individual neurons.

Next, each patient was examined to see what stimuli specific neurons respond to—for example, images of animals, famous people, and objects. The experiment then presented the participant with two familiar images, and they were asked to actively observe one and memorize the other for later (i.e., keep it in working memory but out of attention). Then, the activity of the neurons encoding this "unused" image was examined.

They found that even cells responsible for the image that was not the focus of attention maintained the activity of certain neurons—although their mode of operation was different from when the image was actively observed. A shift in attention resulted in a shift in the pattern of activity, but not its extinction.

This finding challenges the "silent activity" theory and supports the view that information in working memory is actively stored even when not currently in use, Dr. Kamiński said.

The study sheds new light on the mechanisms of information storage in the human brain and may be important in the treatment and diagnosis of disorders in which working memory is impaired – such as ADHD, OCD, schizophrenia or depression.

- Our results significantly deepen the understanding of memory processes and may contribute to the development of better methods of diagnosis and treatment of cognitive disorders - concludes Jan Kamiński.

Link to source article: "Nature Human Behavior".

Ludwik Tomal (PAP)

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