Most individuals remember emotional events, like their wedding day, very clearly, but researchers should not sure how the human brain prioritizes emotional events in memory. In a study published January 16, 2023, by Nature Human Behaviour, Joshua Jacobs, associate professor of biomedical engineering at Columbia Engineering, and his team identified a particular neural mechanism within the human brain that tags information with emotional associations for enhanced memory. The team demonstrated that high-frequency brain waves within the amygdala, a hub for emotional processes, and the hippocampus, a hub for memory processes, are critical to enhancing memory for emotional stimuli. Disruptions to this neural mechanism, brought on either by electrical brain stimulation or depression, impair memory specifically for emotional stimuli.

Rising prevalence of memory disorders

The rising prevalence of memory disorders similar to dementia has highlighted the damaging effects that memory loss has on individuals and society. Disorders similar to depression, anxiety, and post-traumatic stress disorder (PTSD) can even feature imbalanced memory processes, and have develop into increasingly prevalent through the COVID-19 pandemic. Understanding how the brain naturally regulates what information gets prioritized for storage and what fades away could provide critical insight for developing recent therapeutic approaches to strengthening memory for those vulnerable to memory loss, or for normalizing memory processes in those vulnerable to dysregulation.

“It’s easier to recollect emotional events, just like the birth of your child, than other events from around the identical time,” says Salman E. Qasim, lead creator of the study, who began this project during his PhD in Jacobs’ lab at Columbia Engineering. “The brain clearly has a natural mechanism for strengthening certain memories, and we desired to discover it.”

The problem of studying neural mechanisms in humans

Most investigations into neural mechanisms happen in animals similar to rats, because such studies require http://jacobslab.org/direct access to the brain to record brain activity and perform experiments that show causality, similar to careful disruption of neural circuits. However it is difficult to look at or characterize a fancy cognitive phenomenon like emotional memory enhancement in animal studies.

To review this process directly in humans. Qasim and Jacobs analyzed data from memory experiments conducted with epilepsy patients undergoing direct, intracranial brain recording for seizure localization and treatment. During thse recordings, epilepsy patients memorized lists of words while the electrodes placed of their hippocampus and amygdala recorded the brain’s electrical activity.

Studying brain-wave patterns of emotional words

By systematically characterizing the emotional associations of every word using crowd-sourced emotion rankings, Qasim found that participants remembered more emotional words, similar to “dog” or “knife,” higher than more neutral words, similar to “chair.” When taking a look at the associated brain activity, the researchers noted that every time participants successfully remembered emotional words, high-frequency neural activity (30-128 Hz) would develop into more prevalent within the amygdala-hippocampal circuit. When participants remembered more neutral words, or failed to recollect a word altogether, this pattern was absent. The researchers analyzed this pattern across a big data set of 147 patients and located a transparent link between participants’ enhanced memory for emotional words and the prevalence of their brains of high-frequency brain waves across the amygdala-hippocampal circuit.

“Finding this pattern of brain activity linking emotions and memory was very exciting to us, because prior research has shown how necessary high-frequency activity within the hippocampus is to non-emotional memory,” said Jacobs. “It immediately cued us to think in regards to the more general, causal implications-;if we elicit high-frequency activity on this circuit, using therapeutic interventions, will we give you the chance to strengthen memories at will?”

Electrical stimulation disrupts memory for emotional words

With the intention to establish whether this high-frequency activity actually reflected a causal mechanism, Jacobs and his team formulated a novel approach to duplicate the form of experimental disruptions typically reserved for animal research. First, they analyzed a subset of those patients who had performed the memory task while direct electrical stimulation was applied to the hippocampus for half of the words that participants needed to memorize. They found that electrical stimulation, which has a mixed history of either benefiting or diminishing memory depending on its usage, clearly and consistently impaired memory specifically for emotional words.

Uma Mohan, one other PhD student in Jacobs’ lab on the time and co-author on the paper, noted that this stimulation also diminished high-frequency activity within the hippocampus. This provided causal evidence that–by knocking out the pattern of brain activity that correlated with emotional memory–stimulation was also selectively diminishing emotional memory.

Depression acts similarly to brain stimulation

Qasim further hypothesized that depression, which might involve dysregulated emotional memory, might act similarly to brain stimulation. He analyzed patients’ emotional memory in parallel with mood assessments the patients took to characterize their psychiatric state. And, the truth is, within the subset of patients with depression, the team observed a concurrent decrease in emotion-mediated memory and high-frequency activity within the hippocampus and amygdala.

“By combining stimulation, recording, and psychometric assessment, they were in a position to show causality to a level that you simply don’t at all times see in studies with human brain recordings,” said Bradley Lega, a neurosurgeon and scientist on the University of Texas Southwestern Medical Center and never an creator on the paper. “We all know high-frequency activity is related to neuronal firing, so these findings open recent avenues of research in humans and animals about how certain stimuli engage neurons in memory circuits.”

Next steps

Qasim, who’s currently a postdoctoral researcher on the Icahn School of Medicine at Mt. Sinai, is now pursuing this avenue of research by investigating how individual neurons within the human brain fire during emotional memory processes. Qasim and Jacobs hope that their work may additionally encourage animal research exploring how this high-frequency activity is linked to norepinephrine, a neurotransmitter linked to attentional processes that they theorize is perhaps behind the improved memory for emotional stimuli. Finally, they hope that future research will goal high-frequency activity within the amygdala-hippocampal circuit to strengthen and protect memory -; particularly emotional memory.

Our emotional memories are probably the most critical facets of the human experience, informing all the things from our decisions to our entire personality. Any steps we will take to mitigate their loss in memory disorders or prevent their hijacking in psychiatric disorders is hugely exciting.”

Salman E. Qasim, lead creator of the study

Source:

Columbia University School of Engineering and Applied Science

Journal reference:

Qasim, S.E., et al. (2023) Neuronal activity within the human amygdala and hippocampus enhances emotional memory encoding. Nature Human Behaviour. doi.org/10.1038/s41562-022-01502-8.