Image source @ Visual China
Wen 丨 Academic Headlines, Author 丨 Liu Fang, Editor 丨 Huang Shan
Remember the amnesiac girl played by Drew Barrymore in 50 First Loves?
As a result of a car accident, she would forget all about the previous day when the sun rose the next day, so much so that Adam Sandler needed to give her fifty first loves to get her back in love every day.
This sounds like an extremely romantic story, but for people who have lost the function of episodic memory, forgetting is a personal pain.
So, how exactly do humans synthesize episodic memory? What activities does our brains do when we recall the past?
Figure | 50 first dates stills
Recently, from UT Southwestern Peter O'Donnell Jr. Researchers at the Brian Institute found 103 special neurons in the hippocampus region of the human brain that play a key role in extracting episodic memories. The discovery could point the way to new deep brain stimulation (DBS) therapy that could benefit patients with traumatic brain injury (TBI), Alzheimer's and mood disorders.
On December 15, the paper will be published in the journal Science Direct under the title Neurons in the human medial temporal lobe track multiple temporal contexts during episodic memory processing.
Dr Bradley Lega, one of the paper's lead authors, said: "This is the clearest evidence to date on how memories are formed and extracted. ”
Explore the episodic memory of the human brain
First, let's take a look at what Episodic memory is.
Episodic memory refers to the memory of an individual's personal experience of events that occur at a certain time and place. Memories of the first kiss, or of drinking with friends at Christmas, are episodic memories. Another concept, semantic memory, refers to the general information and facts that our brains can store.
Episodic memory belongs to the category of distant memory, which is the most advanced and late mature memory system of human beings. A large body of experimental evidence shows that medial temporal lobe (MTL) plays a major role in situational processing. The brain MTL structure consists of the hippocampus and its surrounding areas (parahius, inner olfactory cortex, olfactory cortex). It can aggregate perceptual information from the pillow, top, temporal, and frontal cortex, and integrate this information.
Schematic diagram of memory encoding and extraction experiments | Source: Papers
In the experiment, neuroscientists recruited 27 patients diagnosed with intractable epilepsy at Thomas Jefferson University Hospital (TJUH) and the University of Texas Southwest (UTSW). Before surgery, they needed to implant microelectrodes in the brain to find epilepsy foci, which provided scientists with neuron-scale experimental data to study the coding patterns of episodic memory.
The memory encoding experiment is a word association task, and 308 different English terms appear on the screen.
Each word appears only once during the experiment for 1.6 seconds on the screen. The subjects then entered a period of interference.
The researchers randomly gave the subjects some arithmetic problems to distract them. Then it's time for exciting memory retrieval. Subjects were required to recall as many of these 308 fleeting English terms as possible. Based on the electrode records of neurons, the researchers eventually identified 103 "Subsequent memory effect" (SME) neurons with significant characteristics of firing frequency when coded successfully, including 51 anterior hippocampal neurons (49.5 percent), 31 posterior hippocampal neurons (30.1 percent), and 4 (3.9 percent) neurons with or without specifying a hippocampal neuron in the hippocampal region and entorhinal cortex of the brain. ), 17 neurons in the olfactory cortex (16.5%).
Schematic diagram of SME neurons | Source: Papers
When memory encoding is successful, the activity of these 103 memory-sensitive neurons increases.
When subjects tried to extract memories, especially recalling large amounts of detail, these neurons produced the same firing pattern. In other words, the neuronal firing frequency of the medial temporal lobe when the subject encoded the word was highly consistent with the firing frequency when recalling the word, and the encoding pattern of each word was different.
The key to discerning memory and hallucinations
This result confirms the "sequential memory effect" theory of the human brain from the scale of neurons, that is, the brain nervous system, especially the medial temporal lobe structure and prefrontal lobe activity, can predict to a certain extent whether the events experienced can be correctly recalled, and the brain activity corresponding to memory success and memory failure is significantly different.
Interestingly, the order in which these words appear rather than semantically is important for recall extraction.
Neurologically this is called the temporal cluster effect, in which the brain tends to encode information adjacent in time. Therefore, when we successfully recall the past, we are usually impressed by the scene before and after an event. In contrast, the 103 memory-sensitive neurons identified this time failed to predict how different semantic types of memories are extracted. For this reason, the researchers specifically found 100,000 documents from Wikipedia to quantify the semantic information given by the words in the experiment.
The authors suggest that this may be because the experimental setup did not require subjects to extract memories by category, or because semantically sensitive neuronal activity was outside the brain's MTL regions, such as the temporal lobe and prefrontal cortex.
(Source: UT Southwestern Peter O'Donnell Jr. Brian Institute website)
In addition, the study also found for the first time in the human brain that the firing time of the neural meridians responsible for extracting memories is different from the firing time of other neurons in the brain. They named this phenomenon "phase offset."
Researchers believe that it is the "phase difference" that allows us to clearly distinguish between memory and reality.
Dr Bradley Lega said: "Our findings provide important implications for this question, namely how do you know you are recalling the past rather than experiencing new memories? ”
The findings are particularly important for Alzheimer's and schizophrenic patients.
Carol Tamminga, a professor of psychiatry at UT Southwestern, said that dysfunction of the hippocampus is the root cause of the inability of people with schizophrenia to distinguish between memory and hallucinations: "The hallucinations and delusions [of patients with schizophrenia and others] are actually real memories that have been damaged. They are processed through the neural memory system, just like 'normal' memory. It is important to understand how this 'phase difference' mechanism can be used to correct these damaged memories. ”
It is expected that in the near future, this major discovery will help people who are troubled by hallucinations and reality.
bibliography:
https://www.sciencedirect.com/science/article/pii/S1053811921009629?via%3Dihub
https://www.sciencedaily.com/releases/2021/12/211206215953.htm
http://icanbrainlab.bnu.edu.cn/webpic/image/20180910/20180910175322.pdf