Life Sciences Database/Wikimedia Commons
The left and right amygdala in red. 
Source: Life Sciences Database/Wikimedia Commons

In recent years, neuroscientists have begun to unravel the many mysteries of the left and right amygdala, as well as the interplay between specific regions within each amygdala. Your amygdalae consists of two separate almond-shaped masses of gray matter seated in the underbelly of your left and right cerebral hemispheres. Together, both amygdala (and neurons housed in various amygdaloid microzones) play a role in how each of us regulate a broad spectrum of emotions and respond to various types of stimuli. 

Contrary to popular belief, your amygdala is not simply a “fear center.” As an example, researchers at MIT recently conducted a study on mice which identified that neurons in the back of the amygdala responded to positive stimuli while neurons in the front of the amygdala responded to fearful stimuli.

Now, a new study has identified that the right amygdala may play a more predominant role in fear responses or someone having an aversion to specific unpleasant stimuli. The latest research on the right amygdala (which is soon to be published) will be presented for the first time by Joel Pieper of the University of California, San Diego at the American Academy of Neurology's Sports Concussion Conference (July 14-16, 2017).

For this amygdala research, Pieper and colleagues studied members of the military with mild-traumatic brain injury (mTBI) who were still active or retired from duty and had been identified with—or without—significant post-traumatic stress disorder (PTSD). The ultimate goal of the study was to identify differences in amygdala volume in patients with partial or full-blown PTSD.  

Notably, those with PTSD displayed larger amygdala volumes overall—with significantly more amygdala gray matter volume on the right side, in comparison to a control group. 

In a statement about the findings, Pieper said: “Many consider PTSD to be a psychological disorder, but our study found a key physical difference in the brains of military-trained individuals with brain injury and PTSD, specifically the size of the right amygdala.” 

There is one important caveat: Although the latest research by Pieper et al. identifies a correlation between amygdala size and PTSD, the researchers are quick to point out that these findings don’t necessarily prove that PTSD causes structural changes in the amygdala or vice versa. (Correlation does not mean causation.) 

Various Amygdala Regions Appear to Regulate Different Emotions

The latest findings on the right amygdala and PTSD dovetail with a recent study on mice from the RIKEN-MIT Center for Neural Circuit Genetics. This study pinpointed that neurons towards the front (anterior region) of the amygdala play a role in regulating behaviors tied to negative emotional responses while neurons in the back (posterior region) of the amygdala respond to positive stimuli.

The 2016 study, "Antagonistic Negative and Positive Neurons of the Basolateral Amygdala," was published in the journal Nature Neuroscience. (I reported on this research in a Psychology Today blog post, "Your Amygdala May House Both Positive and Negative Memories.")

Interestingly, the MIT researchers found that pleasant experiences, tastes, and smells appear to be housed in specific neurons in the back of the basolateral nucleus (BLA). On the flip side, unpleasant, traumatic, and fearful memories appeared to be stored in specific neurons towards the front of the BLA. The researchers concluded that the basolateral amygdala plays an important role in associations linked to both positive and negative stimuli—as well as subsequent emotion regulation or various response behaviors.

Taken together, these human and animal studies could potentially change the way that clinicians approach the diagnosis and treatment of post-traumatic stress disorder. But, more research is needed. Especially because Pieper's PTSD research focuses on mTBI. Future studies by his team will seek to identify if there is a link between mild traumatic brain injury inflicted by blast exposure in combat or through sports-related concussions and someone’s risk of developing PTSD compared to controls without mTBI. 

Hopefully, researchers will soon be able to hone in on how specific regions of the amygdala are linked to PTSD and fine-tune better ways to screen those of us who are at higher risk of developing PTSD. This ongoing amygdala research could also lead to better treatments for the millions of people currently living with post-traumatic stress disorder. Stay tuned for upcoming developments. 

References

Joel Pieper, Sarah Mahasin, Dewleen Baker, Sharon Nichols, Roland Lee, Anne Marie, Angeles Quinto, Ashely Robb Swan, Douglas G. Chang, Mingxiong Huang. "Co-morbid PTSD with mTBI is associated with increased amygdala volume compared to mTBI alone in military-trained individuals." (Presented at the American Academy of Neurology's Sports Concussion Conference in Jacksonville, Fla., July 14 to 16, 2017)

Kim, Joshua, Michele Pignatelli, Sangyu Xu, Shigeyoshi Itohara, and Susumu Tonegawa. "Antagonistic negative and positive neurons of the basolateral amygdala." Nature Neuroscience 19, no. 12 (2016): 1636-1646. DOI: 10.1038/nn.4414

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