Stress influences what we learn and remember. The hormone cortisol, which is released during stressful situations, can make emotional memories in particular stronger. But how exactly does cortisol help the brain build emotional memories?
In a new study, Yale researchers investigated just that. Specifically, they wanted to know how cortisol acts separately on brain circuits that track emotion and those that track memory. They found that cortisol not only helped people remember emotional experiences but also enhanced emotional memory by changing the dynamic brain networks associated with both memory and emotion.
"We all experience stress, and my lab is interested in understanding how stress can be helpful," said corresponding author Elizabeth Goldfarb, an assistant professor of psychiatry at Yale School of Medicine and of psychology in the Faculty of Arts and Sciences.
The findings are published in the journal Science Advances.
While it is known that stress - and cortisol - can help form stronger emotional memories in humans and rodents alike, certain parts of the brain, like the amygdala, are necessary for these benefits. But, because these brain regions can be involved in multiple cognitive processes, it's been difficult to understand how exactly cortisol helps the brain build emotional memories.
"Forming memories for emotional experiences involves different processes in the brain: first, perceiving an experience as emotional or intense, and second, encoding that experience into long-term memory," said Goldfarb, who is also a member of Yale's Wu Tsai Institute, an interdisciplinary research initiative that explores cognition.
For this study, the researchers ran an experiment during which people took a pill containing either hydrocortisone or a placebo before viewing pictures while a functional MRI (fMRI) scan was performed. This type of MRI scan can track oxygenated blood flow to "see" brain activity.
While the participants were viewing these pictures, they told the researchers how they felt about each one. The next day, researchers tested their memory of the pictures. Each participant underwent the study once with hydrocortisone and once with placebo. It was a double-blind design, so neither the participants nor the experimenter knew which pill they had taken.
According to Goldfarb, the key part of this study was how the research team analyzed the data from the fMRI scans. A lot of research uses functional connectivity - or the extent to which responses in one part of the brain are synchronized with those in another part of the brain - to understand behavior. Usually, these analyses require several minutes of brain scan data.
For this study, the researchers looked at functional connectivity during a single trial (around five seconds) and used it to try to predict how people felt about that trial. They also used functional connectivity during a single trial to try to predict whether people remembered that trial the next day. Both of these predictions were successful. So, the researchers were able to define separate dynamic brain networks for memory formation and for emotional intensity and look at how these networks changed with cortisol.
Through their methods, the researchers discovered two things. First, as expected, cortisol helped people remember emotional experiences. Second, cortisol enhanced emotional memory by changing brain networks associated with both memory and emotion. For emotion, cortisol made networks more consistent and more strongly engaged. For memory, cortisol made networks more specialized to emotional content. Cortisol also increased coordination between networks processing emotion and memory.
The findings, researchers say, suggest that multiple dynamic brain mechanisms allow us to selectively remember emotional experiences under stress.
"Stress responses are fundamentally adaptive and can help you form strong memories - but this is specific to experiences that you find to be emotionally intense or meaningful," Goldfarb said. "This is in part related to your brain increasing engagement of networks that track emotional intensity. So, if you are stressed out and trying to learn something new, it might help to think about exciting elements or strong feelings connected to what you are learning."
Other Yale authors include Rajita Sinha, the Foundations Fund Endowed Professor in Psychiatry and professor in the Child Study Center and of neuroscience at Yale School of Medicine (YSM), and Todd Constable, the Elizabeth Mears and House Jameson Professor of Radiology and Biomedical Imaging and professor of neurosurgery at YSM.
The study was supported by grants from the National Institutes of Health.
