EASY-TO-UNDERSTAND SUMMARY:
THE SURVIVAL BRAIN AND ITS CONNECTIONS IN THE AFTERMATH OF TRAUMA
What part of the survival brain plays a key role in how a person responds to trauma?
As described in other areas of this website, PTSD typically includes four types of symptoms: unwanted memories and/or flashbacks of a traumatic event, efforts to avoid reminders of the trauma, emotional distress, and overarousal/hypervigilance. In more recent years, researchers and clinicians have been able to identify two different presentations of PTSD – each linked with a different approach to survival.
Someone with the classic form of PTSD typically experiences active responses meant to help a person fight off/flee from a threat. In contrast, someone with the dissociative subtype of PTSD often experiences passive defense reactions – a sense of disconnection from one’s own body, feelings, and surroundings, which can help a person endure a trauma when fighting or escaping may not be possible. We are interested in understanding any differences in brain connections (i.e., how the brain communicates within itself) when comparing the classic form and dissociative subtype of PTSD.
Research suggests that a small area found deep within the brain, called the periaqueductal gray (PAG), plays a key role in how a person responds to a trauma – e.g., either with the more active responses associated with the classic form of PTSD, or the more passive responses associated with the dissociative subtype of PTSD. The PAG helps connect the upper, “thinking and reasoning” parts of the brain, to lower brain areas that are responsible for more basic body functions (e.g., heart rate, breathing, automatic reactions). It helps guide our behaviour in response to both internal (e.g., pain) and external (e.g., danger) stressors. The PAG is part of the oldest part of the brain, often called the survival brain or the reptilian brain.
The following research explores the role of the PAG in PTSD symptoms by using information from fMRI brain scans (functional magnetic resonance imaging).
How do connections to this part of the survival brain differ in people with classic PTSD, dissociative PTSD, and people without PTSD?
Harricharan S, Rabellino D, Frewen PA, Densmore M, Théberge J, McKinnon MC, Schore AN, & Lanius RA. (2016). fMRI functional connectivity of the periaqueductal gray in PTSD and its dissociative subtype. Brain and Behavior, 6: e00579.
For this study, we used “resting state” fMRI brain scans to help us investigate the role of a brain area called the PAG (periaqueductal grey) in both the classic and dissociative subtype of PTSD. Here, we compared brain images from participants with either classic or dissociative PTSD, to those of people with no mental health diagnoses (the “control” group). A resting state fMRI scan is one during which participants are asked to let their minds wander naturally, so that they’re not reflecting on any one thing in particular; meanwhile, the fMRI scan captures how their brain is functioning.
Knowing that the PAG contains different subsections that are associated with either active defense reactions (i.e., fight/flight), or more passive defence reactions (i.e., dissociation), we expected to find differences in brain activity between our study groups during their resting state scans…. and we were right! It turns out that participants in our three research groups “rested/mind wandered” very differently.
In comparison to our control group, participants in both PTSD groups showed greater connection between parts of the PAG and other brain areas used in emotional and defensive reaction (i.e., brain areas that help someone fight back). In addition to these differences, people in the dissociative PTSD group showed a specific pattern – their brains had greater connection between one area of the PAG and other brain areas linked to passive coping techniques (e.g., dissociation, freezing).
Results of this study suggest that even at rest, the brains (and PAGs) of people with PTSD are preparing to defend themselves and/or cope. Additionally, this suggests that detecting a certain brain connection could help doctors diagnose someone with the dissociative subtype of PTSD. By understanding these brain differences, doctors may be able to select more personalized and effective treatments for PTSD patients.
How is the brain involved in defensive responses in PTSD?
Terpou BA, Harricharan S, McKinnon MC, Frewen P, Jetly R, Lanius RA. The effects of trauma on brain and body: A unifying role for the midbrain periaqueductal gray. J Neuro Res. 2019; 97:1110–1140. https ://doi.org/10.1002/jnr.24447
Hypervigilance involves being on guard for danger, and it is a key feature of PTSD that often leads to an increase in defensive responses or behaviours (e.g., fight, flee, freeze). The specific type of defensive behaviour made, however, can differ depending on the type of PTSD the person is experiencing (e.g., classic or dissociative). In the classic type of PTSD, an individual is more likely to engage in the active responses needed for fight or flight (e.g., increased heartrate, muscle tension – responses that can help someone physically escape from danger). This type of response is triggered by the sympathetic nervous system.
In contrast, people experiencing the dissociative subtype of PTSD tend to experience responses that are more passive in nature, often triggering symptoms of disconnection from one’s own body (e.g., feeling disconnected from their body and/or feelings – responses that might help someone endure or survive a traumatic experience when escape might not be possible). These responses are triggered by the parasympathetic nervous system.
In this paper, we look at the “defense cascade” - a series of automatic reactions in the brain and body triggered by a sense of danger. These reactions are meant to increase the chance of survival. The defense cascade includes responses such as changes in muscle tension, breathing rate, and pain perception. Our goal in writing this paper was to summarize existing research investigating the different types of defensive responses typical in PTSD, in particular, those that involve lower‐level brain areas (including a structure called the periaqueductal grey, or PAG).
In part, certain theories suggest that classic PTSD involves the brain having under-controlled activity in the amygdala (an area of the brain important for intense emotions, such as fear and reward). This poor control leads to the amygdala over-reacting to danger. In contrast, the dissociative subtype of PTSD seems to reflect over-controlled activity in the amygdala. In this case, the strong control causes the amygdala to under-react, resulting in physical and emotional “shut down”. Through our review of the literature, we apply the defense cascade theory to better understand defensive responses involved in PTSD.
