Summary: When hippocampal firing rates are high before exposure to a learning task, people are better able to successfully encode memory. Findings suggest the hippocampus may have a “ready to encode” mode that facilitates memory recall.
What happens in the hippocampus even before people attempt to form memories may impact whether they remember.
A new study analyzed neuronal recordings from the brains of epilepsy patients while they committed a series of words to memory. When the firing rates of hippocampal neurons were already high before the patients saw a word, they were more successful in encoding that word and remembering it later.
The findings suggest that the hippocampus might have a “ready-to-encode” mode that facilitates remembering. The study, published in the Proceedings of the National Academy of Sciences with University of California San Diego researcher Zhisen Urgolites as first author, also suggests that when hippocampal neurons are not already spiking very much, novel information is more likely to be poorly encoded and later 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 on the paper.
“‘Encoding mode’,” Wixted said, “is more than simply paying attention to the task at hand. It is paying attention to encoding, which selectively ramps up activity in the part of the brain that is the most important for making new memories: the hippocampus. Since we know, based on earlier research, that people can actively suppress memory formation, it might be possible for people to get their hippocampus ready to encode as well. But how one might go about doing that, we just don’t know yet.”
Neuronal recordings from the hippocampus, amygdala, anterior cingulate and prefrontal cortex were collected from 34 epilepsy patients while they underwent clinical monitoring at Barrow Neurological Institute. The experiments were originally performed in Peter Steinmetz’s laboratory between 2007 and 2014 when he was at the institute. The data have since been maintained at the Neurtex Brain Research Institute, where Steinmetz is chief scientific officer, and the present research team is newly analyzing the data.
During the experiments, the patients either saw or listened to a steady stream of words and had to indicate whether each word was novel or a repeat. At first, all the words were novel, but after a while most words repeated.
The researchers calculated the average number of times a neuron fired in response to every word the study participants saw or heard. They also calculated the neuronal firing rates immediately preceding each word. Only the average firing rate in the hippocampus approximately one second before seeing or hearing a word for the first time was important: That neuronal activity predicted whether the participants remembered or forgot the word when it was repeated later on.
“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.
The neuronal activity measured in the amygdala, anterior cingulate, and prefrontal cortex did not predict task performance.
“We think new memories are created by sparse collections of active neurons, and these neurons get bundled together into a memory. This work suggests that when a lot of neurons are already firing at high levels, the neuronal selection process during memory formation works better,” Goldinger said.
Megan Papesh, of New Mexico State University, David Treiman, of Barrow Neurological Institute, and Larry Squire, of the Veterans Affairs Medical Center in San Diego and UC San Diego, also contributed to the work.
Spiking activity in the human hippocampus prior to encoding predicts subsequent memory
Encoding activity in the medial temporal lobe, presumably evoked by the presentation of stimuli (postonset activity), is known to predict subsequent memory. However, several independent lines of research suggest that preonset activity also affects subsequent memory. We investigated the role of preonset and postonset single-unit and multiunit activity recorded from epilepsy patients as they completed a continuous recognition task. In this task, words were presented in a continuous series and eventually began to repeat. For each word, the patient’s task was to decide whether it was novel or repeated. We found that preonset spiking activity in the hippocampus (when the word was novel) predicted subsequent memory (when the word was later repeated). Postonset activity during encoding also predicted subsequent memory, but was simply a continuation of preonset activity. The predictive effect of preonset spiking activity was much stronger in the hippocampus than in three other brain regions (amygdala, anterior cingulate, and prefrontal cortex). In addition, preonset and postonset activity around the encoding of novel words did not predict memory performance for novel words (i.e., correctly classifying the word as novel), and preonset and postonset activity around the time of retrieval did not predict memory performance for repeated words (i.e., correctly classifying the word as repeated). Thus, the only predictive effect was between preonset activity (along with its postonset continuation) at the time of encoding and subsequent memory. Taken together, these findings indicate that preonset hippocampal activity does not reflect general arousal/attention but instead reflects what we term “attention to encoding.”