Dissociative amnesia?

June 8, 2021

Written by: Vanessa B. Sanchez

Content Warning: This post includes discussion of sexual assault and trauma

Many people are exposed to a traumatic event during their lifetime. Sexual assault, for example, affects over 63,000 people every year¹. Survivors of traumatic events like assaults are often asked to recall details of their experience, for example to aid in police investigations. Even immediately after the event, however, survivors often have trouble recalling details about their experience. Instead, these memories surface days, weeks, or even years later. There has been intense debate over the meaning and accuracy of these “delayed memories,” and what they can teach us about the way the brain deals with intense emotional and/or physical trauma².  

Many scientists have been intrigued by the idea of delayed memories. It is thought that individuals who’ve experienced trauma also experience dissociation or dissociative amnesia^5-7. Dissociation is a psychological defense mechanism where a person essentially “shuts down” or “disconnects” themselves from their conscious awareness in order to avoid the emotional and physical pain that occurs during a traumatic event^3,7.

This odd phenomena of delayed memory and dissociative amnesia has led many scientists over the past decades to search for neurological signatures of trauma-related dissociation in people with PTSD^2,3,7. To do so, neuroscientists used resting state functional magnetic resonance imaging (R-fMRI) to map out which brain regions communicate with each other more or less in individuals who have experienced trauma.

When using R-fMRI on patients with PTSD or trauma-related dissociation, scientists have always focused on the brain region associated with fear, the amygdala. What they’ve found is that the amygdala is in constant sync with a brain region known for regulating emotions and decision making, the prefrontal cortex, which is thought to underly the emotional detachment, depersonalization, and derealization of these patients⁵. In addition, the amygdala has also been found to be synchronized with the default mode network (DMN)^3,5,7.

The DMN is a group of brain regions (like the prefrontal cortex) known to be involved in cognitive processing. For example, the DMN is known to turn OFF during a cognitive demanding task like when you are trying to figure out how much is 20% off is on a $20 T-shirt at JCPenney. On the other hand, the DMN is known to turn ON when you are internally focused, such as “mind wandering”^3,4. Interestingly, R-fMRI studies on people with PTSD have shown that the DMN’s connectivity appears to be altered or irregular (e.g., it is off at rest)^4,5.

To further explore how dysfunctional the DMN is and how it interacts with other brain regions in patients with PTSD, scientists used R-fMRI on patients with PTSD or trauma-related dissociation^2,3,7. These researchers used a mathematical technique called machine learning to model the relationship between the DMN and other brain regions. Their computational model predicted that the DMN and frontal-parietal network (FPN) communicate with each other more in people with PTSD and are associated with dissociative symptoms^5,6,7. It is though that the more these two brain regions communicate with each other, the more severe the dissociative symptoms will be. The FPN is involved in a plethora of brain functions like motor planning and goal-directed behaviors^3,6. Both brain networks are important for our everyday cognition and their interaction has been reported to be important for internally focused goal-directed behavior⁸. In the case of patients with PTSD, this hyperactive connection between the DMN and FPN implies that these individuals are more internally goal-driven⁷. Psychiatrists have speculated that the reason why this may be is because individuals who’ve experienced a trauma may have learned to rely heavily on themselves rather than a caregiver or spouse⁷.

For the first time, psychiatrists can now use these two brain networks as a biomarker to identify and develop better treatment options for individuals who display dissociative symptoms. The more scientists learn about dissociation and how they are connected to forgotten or delayed memories, the better we can help victims to unlock their memories and accurately recall information about their trauma. Ultimately, the more that the field of neuroscience advances, it can even have the power to solve crimes.

References:

1. Department of Justice, Office of Justice Programs, Bureau of Justice Statistics, National Crime Victimization Survey, 2019 (2020). Note: RAINN applies a 5-year rolling average to adjust for changes in the year-to-year NCVS survey data.
2. Kendall, J. (2021, April 6). Forgotten Memories of Traumatic Events Get Some Backing from Brain-Imaging Studies. Scientific American. https://www.scientificamerican.com/article/forgotten-memories-of-traumatic-events-get-some-backing-from-brain-imaging-studies/.
3. Menon, V. (2021). Dissociation by Network Integration.
4. Spreng, R. N., Stevens, W. D., Chamberlain, J. P., Gilmore, A. W., & Schacter, D. L. (2010). Default network activity, coupled with the frontoparietal control network, supports goal-directed cognition. Neuroimage, 53(1), 303-317.
5. Nicholson, A. A., Densmore, M., Frewen, P. A., Théberge, J., Neufeld, R. W., McKinnon, M. C., & Lanius, R. A. (2015). The dissociative subtype of posttraumatic stress disorder: unique resting-state functional connectivity of basolateral and centromedial amygdala complexes. Neuropsychopharmacology, 40(10), 2317-2326.
6. Lanius, R. A., Bluhm, R. L., Coupland, N. J., Hegadoren, K. M., Rowe, B., Theberge, J., … & Brimson, M. (2010). Default mode network connectivity as a predictor of post‐traumatic stress disorder symptom severity in acutely traumatized subjects. Acta Psychiatrica Scandinavica, 121(1), 33-40.
7. Lebois, L. A., Li, M., Baker, J. T., Wolff, J. D., Wang, D., Lambros, A. M., … & Kaufman, M. L. (2020). Large-Scale Functional Brain Network Architecture Changes Associated With Trauma-Related Dissociation. American Journal of Psychiatry, appi-ajp.
8. Spreng, R. N., Stevens, W. D., Chamberlain, J. P., Gilmore, A. W., & Schacter, D. L. (2010). Default network activity, coupled with the frontoparietal control network, supports goal-directed cognition. Neuroimage, 53(1), 303-317.

Cover image by Joel Naren on Unsplash