The Fear Frequency: Researchers Identify Brain Waves Linked to Fear Response

Researchers have made a frightful finding by teasing out the circuits involved in producing a fear response in the brain and identified a type of brain wave that coordinated this reaction.

A white car meanders down an idyllic country lane. It’s a beautiful summer day. The birds are singing in the trees and the sun shines down. While this scene should, in theory, inspire warm, happy feelings, for anyone who has watched a particular advert for German coffee drink K-Fee, fear might be a more appropriate response.

This video, which you can watch below at your peril, is a classic example of a jump scare video, where the white car’s sedate journey is interrupted by a screaming zombie at around the 15 second mark.

 Reacting to this video is a classic example of a fear response. 

If you’ve viewed this video enough, just the sight of those rolling country hills alone will likely inspire a tensing of the muscles and a grinding of the teeth as a fear response sets in.

A deep fear

While the human body’s physiological response to fear has been well-characterized over centuries of research, how fear starts and spreads in our brain has proved a tougher nut to crack.

Although numerous studies in animals have teased out some fear circuitry, the location of fear circuits in deep regions of the brain has proven a significant hurdle. Now, a group of Chinese and German researchers have taken a unique opportunity to dig down into the regions behind terror in the human brain.

The researchers, who shared their findings in a paper in Science Advances, used those previous animal studies as a starting point for their work. One recent investigation in primates showed that a circuit involving the area commonly known as the brain’s emotional center, the amygdala, and a region of the brain responsible for higher thought named the medial prefrontal cortex (mPFC) might underlie fear learning.

But the amygdala, a small brain region named for its almond shape, is tucked away at the base of the brain, where standard electrophysiological techniques struggle to reach. The team were able to assess the amygdala by studying a cohort of patients who had electrodes implanted in their brain as part of a treatment program for refractory epilepsy.

Scares through squares

These 13 volunteers were subjected to a fear learning task, although not one involving scary coffee commercials. Instead, they were asked to look at either a red or green square on a screen. Randomly, one color of square was paired with a mild electric shock delivered to the volunteers’ wrists.

Over time, this approach would, the researchers hoped, condition the volunteers’ brains to associate a colored square with the fear of being shocked. The team then analyzed, using readouts from the electrode implants, what type of brain signals were sent out from their volunteers’ brain circuits when they were shown a fear-associated square or a non-fear associated square.

The researchers noted that, when a fear-associated square was shown, activity of a type of brain rhythm called a theta wave spiked in both the mPFC and the amygdala. This activity didn’t appear when a non-fear associated square was shown.

Intriguingly, the researchers were able to identify a handful of volunteers whose brains didn’t learn to associate their conditioned square with fear. The team measured these fear reactions by assessing changes in their volunteers’ skin conductance responses, a widely used technique that assesses minute secretions on the skin of volunteers that are indicative of emotional activity. In these “fearless” volunteers, theta activity didn’t spike.

By timing this theta activity, the researchers showed that the activity in the amygdala was driven by earlier activity in a subsection of the mPFC called the dorsal mPFC – a region that was also implicated in fear activity in earlier primate and rodent studies. The correlation between animal and human was not entirely consistent, however – the researchers had hypothesized, based on rodent studies, that another region called the ventral mPFC (vmPFC) would not be involved in encoding fear activity. The vmPFC nonetheless saw theta wave spikes, which the authors suggest may be due to a more complex role for the area in emotional processing that is yet to be fully understood.

Put together, the findings answer some pressing questions about what happens in your brain when that white car starts trundling down the road. But the real value of the work, say the authors in their paper, could be to helping people with post-traumatic stress disorder (PTSD) or anxiety disorders who suffer from an out-of-control fear response.

Reference:

Chen, S, Tan, Z, Xia, W et al.Theta oscillations synchronize human medial prefrontal cortex and amygdala during fear learning. Sci. Adv. 2021;7. doi: eabf4198