Experiences are classified through a system of “boundaries” which marks their beginning and end and makes them recoverable. Understanding the mechanisms of memory is also useful for dealing with degenerative diseases such as Alzheimer’s
But how does the human brain separate, store and retrieve memories? One answer comes from a group of researchers who have identified two types of cells that store memories separately based on how, where and when they occurred. This discovery helps understand how the human brain organizes memory and could have applications for treating ailments such as Alzheimer’s disease. The study, supported by the US National Institutes of Health (NIH) Brain Research through Advancing Innovative Neurotechnologies (BRAIN) Initiative, was published in Nature Neuroscience. “This work transforms the way researchers study the ‘mechanism’ of thinking in the human brain,” says Jim Gnadt, director of the National Institute of Neurological Disorders and Stroke and the BRAIN initiative. “It brings an approach previously used only in non-human primates and rodents to human neurosearch, recording directly from neurons that are generating thoughts.”
Waking state
This study, led by Ueli Rutishauser, a neurosurgeon and neurologist at Cedars-Sinai Medical Center in Los Angeles, began with a simple and naïve question: How does the human brain form and organize memories? Man experiences the waking state as a continuous experience, but many studies of human behavior suggest that these life events are stored as individual and distinct moments. So what marks the beginning and the end of a memory? This theory is called “event segmentation” and there is relatively little knowledge about how this process works in the human brain. To deepen this knowledge, Rutishauser and colleagues worked with 20 patients undergoing intracranial recording of brain activity to guide the surgical approach to treat their drug-resistant epilepsy.
Borders
The researchers observed how patients’ brain activity was affected when they were shown movie clips containing different types of ‘cognitive boundaries’, that is, transitions designed to trigger changes in the way a memory is stored and which mark the onset and the end of memory “files” in the brain. The first type, referred to as a “soft border”, is a video containing a scene which is then cut to another scene that continues the same story. For example, you film a baseball game showing a pitch and, when the batter hits the ball, the camera switches to a “long” shot that captures the rest of the game. On the contrary, a “rigid boundary” is the cut on a completely different story: to stick to the baseball metaphor, it is as if the action of the hitter were immediately followed by an advertisement. Jie Zheng, a fellow at Boston Children’s Hospital and first author of the study, explains the fundamental difference between the two boundaries. “Is it a new scene within the same story or two completely different stories? Changing the narrative from one clip to the next determines the type of cognitive boundary, ”Zheng explains. The researchers recorded the participants’ brain activity as they watched the videos and noted that two distinct groups of cells responded to different types of boundaries by increasing their activity. A first group, called “boundary cells” became more or less active in response to a soft or hard boundary. A second group, called “event cells”, responded only to rigid boundaries. This has led to the theory that a new “memory cell” is created only when there is a spike in the activity of both soft boundary and event cells, which only occurs when the boundary is rigid.
Photo method
An analogy to how memories can be stored and made accessible in the brain is the method by which photos are stored on the phone or computer. Often photos are automatically grouped into events based on where and when they were taken, then displayed as key photos from that event. When you tap or click on that photo, you can delve into that specific event. «A response to the border can be thought of as the creation of a new photographic event» says dr. Rutishauser. “As the memory is built, it is as if new photos are added to that event. When a rigid boundary is created, that event is “closed” and a new one opens. Soft borders can be thought of to represent new images created within a single event ». The researchers then looked at memory retrieval and how this process associates with the activation of boundary cells and events. It has been theorized that the brain uses boundary peaks as markers for “scrolling” past memories, just as “key photos” are used to identify each event. When the brain finds a firing pattern that looks familiar, it “opens” that particular event.
The tests
Two different memory tests were designed to test this theory. In the first, the participants were first shown a video, then a series of still images, and were asked whether the images came from a scene from the video they just watched or not. Study participants were more likely to remember images that passed immediately after a hard or soft boundary — when a new “photo” or “event” would be created. The second test consisted of showing pairs of images from videos they had just seen. Participants were then asked which of the two images appeared first. It turned out that it was much more difficult to choose the correct order of the images if the two appeared on different sides of a rigid boundary, perhaps because the brain had associated them with different “events”. These findings provide a glimpse into how the human brain creates and stores memories and then accesses them. Since event segmentation is a process that can be compromised in people with memory disorders, such as Alzheimer’s disease, these insights could be applied to the development of new therapies. In the future, Rutishauser and his team plan to explore two possible avenues for developing therapies related to these outcomes. In the first, neurons that use dopamine as a neuromediator and are best known for their role in reward mechanisms, can be activated by “boundary cells” and “event cells”, suggesting a possible target to help strengthen the formation. memories. In the second, one of the brain’s normal internal rhythms, known as the theta rhythm on electroencephalography (EEG), has been linked to learning and memory. If the event cells were excited in time with that rhythm, the participants could more easily remember the order of the images that had been shown to them. Since deep brain stimulation can affect theta rhythms, this could be another avenue for treating patients with certain memory impairments.
March 22, 2022 (change March 22, 2022 | 17:23)
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