Summary: A study from the University of Pennsylvania has found that electrical signals in the human hippocampus differ immediately before recollection of true and false memories. The researchers found that low-frequency activity in the hippocampus decreases as a function of contextual similarity between a falsely recalled word and the target word. This suggests that the hippocampus stores information about an item with the context in which it was presented.
Let’s assume you usually had eggs for breakfast, but because you were late, you ended up eating cereal. Other contextual parallels persisted as you crunched on a mouthful of Raisin Bran: you were getting ready to leave the same job and eating at the same table at the same time. You recall eating eggs, but when someone asks you later what you had for breakfast, you answer wrong.
This would be a real-world example of a false memory. But what happens in your brain before recalling eggs, compared to what would happen if you correctly recalled cereal?
In a paper published in Proceedings of the National Academy of Sciences, University of Pennsylvania neuroscientists show for the first time that electrical signals in the human hippocampus differ immediately before recollection of true and false memories. They also found that low-frequency activity in the hippocampus decreases as a function of contextual similarity between a falsely recalled word and the target word.
“Whereas prior studies established the role of the hippocampus in event memory, we did not know that electrical signals generated in this region would distinguish the imminent recall of true from false memories,”
said psychology professor Michael Jacob Kahana, director of the Computational Memory Lab and the study’s senior author. He says this shows that the hippocampus stores information about an item with the context in which it was presented.
Lower Theta, Higher Alpha
Researchers also found that, relative to correct recalls, the brain exhibited lower theta and high-frequency oscillations and higher alpha/beta oscillations ahead of false memories. The findings came from recording neural activity in epilepsy patients who were already undergoing invasive monitoring to pinpoint the source of their seizures.
Noa Herz, lead author and postdoctoral fellow in Kahana’s lab at the time of the study, explains that the monitoring was done using intracranial electrodes, which was the methodology researchers wanted to use for this study. She says that, compared to scalp electrodes, this method allowed them to more precisely and directly, measure the neural signals that were generated in deep brain structures, so the activity we are getting is much more localized.
Subjects studied a list of unrelated words and were distracted before being asked to remember words, says Herz, now an assistant professor of neurology at Thomas Jefferson University, in Philadelphia. Researchers analyzed patterns of electricity generated in the hippocampus, capturing brain activity leading up to correct or false recall.
Genuine vs. False Memories
Researchers hypothesized that hippocampal activity would represent the degree of similarity between the true and false memory, in addition to the difference between genuine and false memories. In fact, they discovered that there was a stronger correlation between the settings in which incorrect and accurate items were learned and a noticeable decrease in low-frequency activity.
A similar context in this study meant a patient recalling a word from a prior list in the experiment instead of the target list, whereas a different context is recalling a word that was never part of the experiment.
“The words were presented when the patient was sitting in the same room, looking at the same computer, having the same experimenter next to him,” Herz said, “and these words were also presented more recently in time, so all of these different factors mean that prior list intrusions should be more similar, in terms of the context in which they were presented, to the correct target list.”
This method was used to assess the hippocampus response to various words presented from comparable source contexts, but what does the brain do when a person remembers a word that is semantically similar to the correct phrase but is incorrect? Researchers also tested this.
Similar But Wrong
They showed patients words in three categories, such as flowers, fruits, and insects. As an example, Herz said if the list includes “rose” and “lily,” but a person recalls “sunflower,” that is semantically similar, whereas saying “clock” is not.
But perhaps “clock” was on a prior word list in the study; the paper notes a recalled word tends to be similar in at least one context, either source or semantic.
As hypothesized, researchers found the same brain pattern with semantic similarity as they did with source similarity: a reduction in hippocampal low-frequency activity.
Herz says the overall findings deepen the understanding of how the brain enables memory retrieval, and the authors note that predicting false memories at a single-subject level is particularly important when false recalls cause distress.
Individuals suffering from stress-related psychopathology, such as post-traumatic stress disorder, often experience memory intrusions of their traumatic experiences under contexts that are safe and dissimilar to the traumatic incident. Targeted interventions that disrupt retrieval of intrusive memories could spawn novel therapies for such clinical conditions,”
the researchers write.
Abstract
Failure of contextual retrieval can lead to false recall, wherein people retrieve an item or experience that occurred in a different context or did not occur at all. Whereas the hippocampus is thought to play a crucial role in memory retrieval, we lack understanding of how the hippocampus supports retrieval of items related to a target context while disregarding related but irrelevant information. Using direct electrical recordings from the human hippocampus, we investigate the neural process underlying contextual misattribution of false memories. In two large datasets, we characterize key physiological differences between correct and false recalls that emerge immediately prior to vocalization. By differentiating between false recalls that share high or low contextual similarity with the target context, we show that low-frequency activity (6 to 18 Hz) in the hippocampus tracks similarity between the current and retrieved context. Applying multivariate decoding methods, we were able to reliably predict the contextual source of the to-be-recalled item. Our findings elucidate one of the hallmark features of episodic memory: our ability to distinguish between memories that were formed on different occasions.
Reference:
- Herz N, Bukala BR, Kragel JE, Kahana MJ. Hippocampal activity predicts contextual misattribution of false memories. Proc Natl Acad Sci U S A. 2023;120(40):e2305292120. doi:10.1073/pnas.2305292120