It’s no easy task to recall what you had for dinner last night or where you left your car this morning. Despite this, the brain region responsible for these types of memories, known as episodic memories, is about the size of a marble. Only the human hippocampus encodes the “what, when, where” information that forms the cornerstone of episodic memories, according to a new study of neural recordings from epilepsy sufferers’ brains. Only a few neurons in the seahorse-shaped hippocampus are involved in recalling an experience like last night’s dinner since this information is sparsely coded.
These findings show that there isn’t much neuronal activity in the hippocampus when we remember; instead, there is a focused, precise, and sparse signal. Stephen Goldinger, an Arizona State University professor of psychology said that this suggests that a small collection of hippocampus neurons — perhaps 2% of the cells they observed, or 50 neurons in total collectively code an episodic memory. He added that they would never find this type of signal using other methods for assessing brain activity in people, such as fMRI. The findings, which were published in the Proceedings of the National Academy of Sciences on May 5, also reveal that the hippocampus collaborates with other brain areas to keep track of broad information such as whether something is new or familiar.
University of California, San Diego, Arizona State University, Barrow Neurological Institute, New Mexico State University, Veterans Affairs Medical Center in San Diego, and the Neurtex Brain Research Institute collaborated on the research. The researchers recorded the brains of 34 epilepsy patients at Barrow Neurological Institute while they were being monitored. As the patients listened to or read a steady stream of words, the research team measured neuronal activity in the hippocampus, amygdala, anterior cingulate, and prefrontal cortex. Some of the words began to repeat themselves after a while. The patients were asked to indicate whether a word was new or had been repeated.
The activity in the hippocampus monitored whether or not the patients had been exposed to a word. Other brain areas had active neurons as well, but they encoded different information. The hippocampal activity we detected was simply a ‘do you remember this or not’ signal. The amygdala, cingulate cortex, and prefrontal cortex were all involved in determining if words were recognisable, but their signals were “apparently constructed upon the ‘what, where, and when’ information contained in the hippocampus,” according to Goldinger. He mentioned that this broad signal in the amygdala, cingulate cortex, and prefrontal cortex suggests that those brain areas index hippocampal-generated memories in completing the task.
Megan Papesh, an ASU alumna now at New Mexico State University, contributed to the project. The paper’s first authors were Zhisen Urgolites and John Wixted of UC San Diego, and senior author Peter Steinmetz of the Neurtex Brain Research Institute. The work was also assisted by David Treiman of Barrow Neurological Institute and Larry Squire of the Veterans Affairs Medical Center in San Diego and UC San Diego. The Neurtex Brain Research Institute, the National Institute of Child Health and Human Development, the National Institute of Neurological and Communicative Disorders and Stroke, the National Institute of Mental Health, and the Department of Veterans Affairs’ Medical Research Service all contributed to this research.
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