Most people remember emotional events, such as their wedding day, very clearly, but researchers aren’t sure how the human brain prioritizes emotional events in memory. In a study published on January 16, 2023, by The nature of human behaviorJoshua Jacobs, Associate Professor of Biomedical Engineering at Columbia University of Engineering, and his team have identified a specific neural mechanism in the human brain that marks information with emotional associations to enhance memory. The team showed that high-frequency brain waves in the amygdala, the center of emotional processes, and the hippocampus, the center of memory processes, are essential to consolidating memory for emotional stimuli. Disturbances in this neural mechanism, caused either by electrical stimulation of the brain or by depression, impair memory specifically for emotional stimuli.
High prevalence of memory disorders
The increasing prevalence of memory disorders such as dementia has highlighted the harmful effects of memory loss on individuals and society. Disorders such as depression, anxiety, and post-traumatic stress disorder (PTSD) can also be characterized by unbalanced memory processes, and have become increasingly prevalent during the COVID-19 pandemic. Understanding how the brain naturally organizes what information takes priority for storage and what withers away could provide insights into developing new therapeutic approaches to strengthen memory for those at risk of amnesia, or to normalize memory processes in those at risk of dysregulation.
“It is easier to remember emotional events, such as the birth of your child, than other events around the same time,” says Salman Qasim, lead author of the study, who began this project during his Ph.D. in Jacobs’ lab at Columbia. engineering. “It’s clear that the brain has a natural mechanism for strengthening certain memoriesAnd And we wanted to get to know him.”
Difficulty studying neurological mechanisms in humans
Most investigations into neural mechanisms are done in animals such as mice, because such studies require it http://jacobslab.org/direct Brain access to record brain activity and perform experiments that show causation, such as subtle disruption of neural circuits. But it is difficult to observe or describe such a complex cognitive phenomenon as the enhancement of emotional memory in animal studies.
To study this process directly in humans. Kasim and Jacobs analyzed data from memory experiments conducted with epilepsy patients undergoing direct intracranial recording to localize and treat seizures. During the recordings, epilepsy patients memorize lists of words while electrodes placed in the hippocampus and amygdala record the brain’s electrical activity.
Examine the brain wave patterns of emotional words
By systematically characterizing the emotional associations of each word using emotion ratings from the audience, Kasem found that participants remembered more emotional words, such as “dog” or “knife,” better than more neutral words, such as “chair.” Looking at the correlated brain activity, the researchers noted that the more successfully the participants recalled the emotional words, the higher-frequency (30-128 Hz) neural activity would become more prevalent in the circuitry of the amygdala and hippocampus. When participants remembered more neutral words, or failed to remember a word completely, this pattern was absent. The researchers analyzed this pattern across a large data set of 147 patients and found a clear link between the participants’ enhanced memory of emotional words and the propagation of high-frequency brain waves in their brains across the circuit of the amygdala and hippocampus.
“Finding this pattern of brain activity that links emotions and memory was very exciting for us, because previous research has shown how important high-frequency activity in the hippocampus is for non-emotional memory,” Jacobs said. “We were immediately prompted to consider the more general causal effects—; if we can elicit high-frequency activity in this circuit, using therapeutic interventions, will we be able to strengthen memories at will?”
Electrical stimulation disrupts the memory of emotional words
In order to determine whether this high-frequency activity actually reflects a causal mechanism, Jacobs and his team formulated a unique approach to replicate the type of experimental perturbations typically reserved for animal research. First, they analyzed a subset of those patients who performed a memory task while applying direct electrical stimulation to the hippocampus for half of the words the participants had to memorize. They found that electrical stimulation, which has a mixed history of either benefiting or diminishing memory depending on its use, visibly and consistently impairs memory particularly for emotional words.
Uma Mohan, another PhD student in Jacobs’ lab at the time and a co-author on the paper, noted that this stimulation also reduced high-frequency activity in the hippocampus. This provided causal evidence that—by eliminating the pattern of brain activity associated with emotional memory—stimulation was also selectively reducing emotional memory.
Depression works similarly to brain stimulation
Kasim further hypothesized that depression, which can involve disorganized emotional memory, may work similarly to brain stimulation. He analyzed the patients’ emotional memory in parallel with the mood assessments that the patients took to describe their psychological state. Indeed, in the subgroup of depressed patients, the team observed a simultaneous decrease in emotion-mediated memory and high-frequency activity in the hippocampus and amygdala.
Not an author on the paper, said Bradley Lega, a neurosurgeon and scientist at the University of Texas Southwestern Medical Center. “We know that high-frequency activity is associated with neuronal firing, so these findings open up new avenues of research in humans and animals on how certain stimuli engage neurons in memory circuits.”
Kassem, who is currently a postdoctoral researcher at the Icahn School of Medicine at Mount Sinai, is now pursuing this avenue of research by investigating how individual neurons in the human brain fire during emotional memory processes. Kasim and Jacobs hope their work will also inspire animal research exploring how this high-frequency activity is related to norepinephrine, a neurotransmitter involved in attentional processes that they hypothesize may be behind the enhanced memory of emotional stimuli. Finally, they hope that future research will target high-frequency activity in the circuit of the amygdala and hippocampus to strengthen and protect memory. -; Especially emotional memory.
Our emotional memories are one of the most important aspects of the human experience, informing everything from our decisions to our entire personality. Any steps we can take to mitigate their loss in memory disorders or prevent their hijacking in psychiatric disorders is very exciting.”
Salman Qasim, lead author of the study
Qassem South et al. (2023) Neural activity in the human amygdala and hippocampus enhances the encoding of emotional memory. The nature of human behavior. doi.org/10.1038/s41562-022-01502-8.