Summary: When hippocampal neurons are already firing at higher rates just before a learning event, people are more likely to encode and later recall that information. The results support the idea of a hippocampal “ready-to-encode” state that improves memory formation.
Source: UCSD
What the hippocampus does before you try to form a memory can determine whether you remember it.
New analyses of direct neuronal recordings from people with epilepsy reveal that the state of the hippocampus immediately before encountering new information predicts whether that information will be stored successfully. In a study led by researchers at the University of California San Diego and published in the Proceedings of the National Academy of Sciences, higher firing rates in hippocampal neurons roughly one second before a word was presented were associated with better encoding and improved likelihood of later recall when the word reappeared.
The pattern supports the notion that the hippocampus can enter a “ready-to-encode” mode. When hippocampal neurons are already active at elevated levels, the brain appears better prepared to bind neurons into a lasting memory trace. Conversely, when hippocampal firing is low prior to an event, novel items are more likely to be poorly encoded and subsequently forgotten.
“A key question going forward is how to put our brains into ‘encoding mode’ when we wish to do so,” said John Wixted, professor of psychology at UC San Diego and one of the lead authors. He emphasized that encoding mode is not merely general attention to a task, but a selective attentional state that boosts activity in the hippocampus—the brain structure most critical for forming new episodic memories. Previous work shows people can sometimes suppress memory formation, so the authors suggest it may someday be possible to deliberately enhance hippocampal readiness for encoding, although the methods to do so are not yet known.
The study reanalyzed recordings originally collected from 34 epilepsy patients who underwent clinical monitoring at Barrow Neurological Institute. The experiments were carried out in Peter Steinmetz’s laboratory between 2007 and 2014; the archived data are maintained at the Neurtex Brain Research Institute. During the continuous recognition task used for this research, patients encountered a steady stream of words—presented visually or auditorily—and indicated whether each word was novel or a repeat. Initially most words were new; over time many items recurred. The research team calculated both peristimulus firing rates and the firing rates that occurred immediately before each word presentation.
The most important finding was very specific: only the average firing rate in the hippocampus about one second before the first presentation of a word predicted whether that word would be remembered later when it was repeated. Other brain regions monitored in the recordings—the amygdala, anterior cingulate cortex, and prefrontal cortex—did not show predictive pre-stimulus activity. Postonset activity during encoding also related to later memory, but it appeared to be an extension of the preonset hippocampal state rather than an independent predictor.
“If a person’s hippocampal neurons were already firing above baseline when they saw or heard a word, their brain was more likely to successfully remember that word later,” said Stephen Goldinger, professor of psychology at Arizona State University. He and other authors note that new memories are thought to arise from sparse sets of neurons that become linked; when many neurons are already active, the selection and bundling processes required to form a stable memory trace may operate more effectively.
The research team included Zhisen J. Urgolites as first author, along with John T. Wixted, Stephen D. Goldinger, Megan H. Papesh, David M. Treiman, Larry R. Squire, and Peter N. Steinmetz. Their analysis highlights a precise temporal window in which hippocampal readiness influences whether encoding leads to later retrieval, and it argues that this effect reflects selective “attention to encoding” rather than general arousal or nonspecific attention.
About this neuroscience research article
Source:
UCSD
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Inga Kiderra – UCSD
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The image is in the public domain.
Original Research: Closed access
“Spiking activity in the human hippocampus prior to encoding predicts subsequent memory” by Zhisen J. Urgolites, John T. Wixted, Stephen D. Goldinger, Megan H. Papesh, David M. Treiman, Larry R. Squire, and Peter N. Steinmetz. PNAS doi: 10.1073/pnas.2001338117
Abstract
Spiking activity in the human hippocampus prior to encoding predicts subsequent memory
Encoding-related activity in medial temporal regions that follows stimulus presentation (postonset activity) is known to predict later memory. However, a range of evidence suggests that activity occurring before stimulus onset can also influence encoding success. To investigate this, the authors examined single-unit and multiunit activity recorded from epilepsy patients performing a continuous recognition task in which words were presented in sequence and eventually repeated. For each item, participants judged whether the word was novel or repeated. The study found that preonset spiking in the hippocampus—specifically when a word was novel—predicted whether that word would be remembered when it later reappeared. Postonset activity during encoding also predicted subsequent memory, but it largely continued the preonset activity. The predictive influence of preonset spiking was much stronger in the hippocampus than in the amygdala, anterior cingulate, or prefrontal cortex. Additionally, preonset and postonset activity around encoding did not predict performance at the time of novelty judgments, nor did preonset and postonset activity around retrieval predict correct recognition of repeated words. In sum, the only reliable predictor observed was preonset hippocampal activity (and its continuation) at encoding, supporting the idea that this activity reflects focused “attention to encoding” rather than general arousal or nonspecific attention.
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