Neurologists at the University of Miami have come closer to understanding the complex role that the amygdalae, a pair of small clusters of nuclei deep within the brain, play in human mood and behavior.

People whose amygdalae show more pronounced responses to distressing experiences in the short term may have a higher risk of reporting negative emotions over long stretches of time, according to an analysis published Monday in the Journal of Neuroscience. These findings could shed light on the mechanisms of the brain that lead to depression and anxiety and pave the way for early interventions in people who are at risk of developing more severe emotional disorders.

The amygdala is often referred to as the fear center of the brain. It can quickly interpret sensory inputs and initiate the fight or flight response when it perceives a potentially dangerous situation. Psychologists use the term "amygdala hijack" to refer to situations in which a person's amygdala responds prematurely to an event that is not actually dangerous.

Although the amygdala is often characterized in popular science as an irrational, primitive area of the brain where split-second decisions give way to foolish behavior, Nikki Puccetti, a doctoral candidate in the University of Miami's clinical psychology program and a lead author of the journal article, isn't as quick to dismiss the region's role.

"The amygdala, in some ways, has been oversimplified," Puccetti told The Academic Times. "It's not just about encoding fear and it's not just about encoding emotion. It's more about evaluating the relevance and the salience and the importance of stimuli in our environment."

In the study, the researchers examined a dataset provided by the University of Wisconsin-Madison's Midlife in the U.S. project, an ongoing, long-term study that surveyed and scanned the brains of thousands of participants starting in the mid-1990s. The Midlife study tracked participants' moods through brain scans, phone interviews and daily diaries in an attempt to connect brain function with everyday experiences and emotions. 

But while the original investigators hadn't singled out the amygdala as a point of interest in the study, the University of Miami neuroscientists realized that they could tap into a subset of data from 52 individuals who participated in functional magnetic resonance imaging scans in order to isolate their amygdala activity. 

Participants were shown negative images interwoven with neutral ones. The neurologists then monitored the degree to which each subject's amygdala remained active after the negative sensory experience had concluded and been replaced by the neutral stimulus.

They discovered that those participants whose amygdala responded disproportionately to the neutral sensory inputs reported higher degrees of negative affect, a term psychologists use to refer to the experiential components of emotions such as anger, guilt and nervousness. 

For those subjects, the amygdala may have shown a bias toward the subsequent neutral stimulus, believing it to be somehow related to the original distressing image. The team's findings were more pronounced in the left amygdala compared to the right. The study showed a 0.37 correlation coefficient between left amygdala persistence and negative affect, representing a moderate linear relationship. Meanwhile, there was a -0.47 coefficient between left amygdala persistence and positive emotions, representing a negative correlation.

Those people whose amygdala was less active during the neutral experience were more likely to report positive emotions.

This might at first glance appear to be an intuitive finding, but Puccetti said that what was more surprising was that these effects seem to persist over the span of years. Thirty-one individuals, for instance, showed relatively constant positive well-being results after seven years, with a correlation of about 0.8. The findings suggest that varying levels of amygdala activity may provide a hidden explanation for why people respond to real-world events with different degrees of negativity or positivity.

Puccetti clarified that the study didn't account for the various substructures that make up the amygdala, which scientists are still working to unravel. "There are actually numerous subnuclei in the amygdala. And they have some overlap in terms of their function, but they're also playing distinct roles in some ways," she said.

Next, Puccetti hopes to replicate the team's recent findings with a new set of participants. She cautioned, though, that the findings should not be overstated, since brain regions rarely work in isolation. In future studies, Puccetti intends to explore how the amygdala plays a role in larger brain systems.

"It's really about this interplay of regions and circuits in the brain that support emotion dynamics," Puccetti said. "It challenges you not to become an expert in a single region, but to really have a deep understanding of the orchestration of different regions in the brain and the different networks that we know about."

The study, "Linking Amygdala Persistence to Real-World Emotional Experience and Psychological Well-Being," published March 22 in the Journal of Neuroscience, was authored by Nikki A. Puccetti and Aaron S. Heller, University of Miami; Stacey M. Schaefer, Carol D. Ryff and Richard J. Davidson, University of Wisconsin-Madison; Carien M. van Reekum, University of Reading; Anthony D. Ong, Cornell University; and David M. Almeida, Pennsylvania State University.