Perceived threat, PTSD and the vagus nerve

The new study, published by researchers at UC San Diego in the journal, Brain Stimulation, has investigated the effect that the nervous system might have on mechanisms of PTSD and trauma in the brain by practicing ‘non-invasive stimulation’ of the vagus nerve in participants who then had a series of brain scans via fMRI.

One of the central mechanisms and symptoms of PTSD, complex PTSD and trauma is that a person’s ability to perceive threat accurately can become dysregulated. This can result in a person being hypervigilant, constantly scanning for danger and can even result in people, such as survivors of abuse, being unable to effectively evaluate danger – becoming over-careful in some situations and then putting themselves in danger at other times.

What is the vagus nerve?

The vagus nerve is one of 12 cranial nerves in the body. The vagus nerve is vitally involved in the autonomic nervous system, most especially the parasympathetic branch of this nervous system. The parasympathetic nervous system is what we often call our ‘resting state’, this is because it governs functions like digestion, breathing, and heart rate. The steady regulation of all three of these factors is often ignored when we are in fight or flight, which is when our sympathetic nervous system is engaged.

How have researchers utilised the important function of the vagus nerve and the parasympathetic nervous system to test functions around fight or flight?

The research team at UC San Diego brought together a group of 24 healthy adults who were to either receive non-invasive stimulation of the vagus nerve, or who would receive a placebo treatment.

During the stimulation of the vagus nerve participants entered an fMRI machine and were asked to complete a simple task, which involved pressing a button whenever a blue circle or square was shown to them, this was to measure their reaction time during vagus nerve stimulation.

The next phase of the study involved monitoring their responses to different stimuli. All participants were told that if the shape turned red, they would immediately be shown an ‘upsetting image’ such as an image of warfare, which would be accompanied by a high-pitched tone, or the shape would turn green to signal a ‘pleasant image’ such as a quiet lakeside, which would accordingly be accompanied by a soothing low tone.

During the study, researchers recorded the difference in participants reaction time, brain activity and blood oxygen levels.

Vagus nerve stimulation repeatedly showed an increased reaction time for both neutral and ‘emotionally charged’ stimuli. Importantly however, researchers discovered that those undergoing vagus nerve stimulation had an increased brain response to the upsetting imagery, and a diminished response to pleasant imagery, while the direct opposite was true for the placebo group.

What does this mean for research into PTSD and treatment?

The study’s lead author, Dr. Imanuel Lerman of UC San Diego’s Qualcomm Institute has acknowledged the possible limits of their findings due to their small sample size in an interview with UC San Diego’s news outlet. However, they emphasised the intriguing nature of their results and what they might point to:

“The stimulation of participants’ vagus nerve heightened their reaction to negative images and decreased reaction to positive images. This supports the idea that there’s an additive link between vagus nerve stimulation and norepinephrine signalling, which is critical for fight or flight responses, in the brain.”

This “addictive link” between vagus nerve stimulation, negative stimuli and the fight or flight response could actually inform professionals on how to treat patients, Dr Lerman said, characterising it as a “first step toward understanding how non-invasive vagus nerve stimulation might be efficiently used as a tool to treat patients with PTSD, generalised anxiety and other disorders that involve a heightened response to received threat.”

Written by Bryony Porteous-Sebouhian

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