Overthinking what you said? It’s your ‘lizard brain’ talking to newer, advanced parts of your brain

• Study sought to better understand how humans evolved to become skilled at thinking about others
• Newer parts of the brain that support social interactions are connected to and in constant communication with the ancient amygdala
• First study to map with fMRI never-before-seen details of the brain’s social cognitive network

CHICAGO --- We’ve all been there. Moments after leaving a party, your brain is suddenly filled with intrusive thoughts about what others were thinking. “Did they think I talked too much?” “Did my joke offend them?” “Were they having a good time?”

In a new Northwestern Medicine study, scientists sought to better understand how humans evolved to become so skilled at thinking about what’s happening in other peoples’ minds. The findings could have implications for one day treating psychiatric conditions such as anxiety and depression.

“We spend a lot of time wondering, ‘What is that person feeling, thinking? Did I say something to upset them?’” said senior author Rodrigo Braga. “The parts of the brain that allow us to do this are in regions of the human brain that have expanded recently in our evolution, and that implies that it’s a recently developed process. In essence, you’re putting yourself in someone else’s mind and making inferences about what that person is thinking when you cannot really know.”

The study found the more recently evolved and advanced parts of the human brain that support social interactions — called the social cognitive network — are connected to and in constant communication with an ancient part of the brain called the amygdala.

Often referred to as our “lizard brain,” the amygdala typically is associated with detecting threats and processing fear. A classic example of the amygdala in action is someone’s physiological and emotional response to seeing a snake: startled body, racing heart, sweaty palms. But the amygdala also does other things, Braga said.

“For instance, the amygdala is responsible for social behaviors like parenting, mating, aggression and the navigation of social-dominance hierarchies,” said Braga, an assistant professor of neurology at Northwestern University Feinberg School of Medicine. “Previous studies have found co-activation of the amygdala and social cognitive network, but our study is novel because it shows the communication is always happening.”

The study was published Nov. 22 in the journal Science Advances.

High-resolution brain scans were key

Within the amygdala, there’s a specific part called the medial nucleus that is very important for social behaviors. This study was the first to show the amygdala’s medial nucleus is connected to newly evolved social cognitive network regions, which are involved in thinking about other people. This link to the amygdala helps shape the function of the social cognitive network by giving it access to the amygdala’s role in processing emotionally important content.

This was only possible because of functional magnetic resonance imaging (fMRI), a noninvasive brain-imaging technique that measures brain activity by detecting changes in blood oxygen levels. A collaborator at the University of Minnesota and co-author on the study, Kendrick Kay, provided Braga and co-corresponding author Donnisa Edmonds with fMRI data from six study participants’ brains, as part of the Natural Scenes Dataset (NSD). These high-resolution scans enabled the scientists to see details of the social cognitive network that had never been detected on lower-resolution brain scans. What’s more, they were able to replicate the findings up to two times in each individual.

“One of the most exciting things is we were able to identify network regions we weren’t able to see before,” said Edmonds, a neuroscience Ph.D. candidate in Braga’s lab at Northwestern. “That’s something that had been underappreciated before our study, and we were able to get at that because we had such high-resolution data.”

Potential treatment of anxiety, depression

Both anxiety and depression involve amygdala hyperactivity, which can contribute to excessive emotional responses and impaired emotional regulation, Edmonds said. Currently, someone with either condition could receive deep brain stimulation for treatment, but since the amygdala is located deep within the brain, directly behind the eyes, it means having an invasive, surgical procedure. Now, with this study’s findings, a much less-invasive procedure, transcranial magnetic stimulation (TMS), might be able to use knowledge about this brain connection to improve treatment, the authors said.

“Through this knowledge that the amygdala is connected to other brain regions — potentially some that are closer to the skull, which is an easier region to target — that means people who do TMS could target the amygdala instead by targeting these other regions,” Edmonds said.

The study is titled, “The human social cognitive network contains multiple regions within the amygdala.” Other Northwestern co-authors include Christina Zelano, Joseph J. Salvo, Nathan Anderson, Maya Lakshman and Qiaohan Yang.

The study was supported in part by National Institute of Mental Health (grant R00 MH117226), an Alzheimer's Disease Core Center grant (P30 AG013854), a pilot award from the National Institute on Aging, a training award (T32 NS047987), the William Orr Dingwall Foundations of Language Fellowship, and National Science Foundation (NSF) GFRP (award DGE-2234667), and National Institute on Deafness and Other Communication Disorders (grant R01DC018539). Collection of the NSD dataset was supported by NSF IIS-1822683 and NSF IIS-1822929.