In PTSD, Brain Loses Ability to Distinguish Safe From Perilous Stimuli, Study Finds

Posted on March 29th, 2016

A study from a team of Indian researchers indicates that the indiscriminate fear often found in people affected by PTSD may be the result of abnormalities in a key brain structure called the amygdala.

People with PTSD (post-traumatic stress disorder) often experience fear and detect danger in situations that most individuals view as low-risk or essentially harmless. In a study published in December 2014 in the journal Nature Neuroscience, a group of researchers from India’s Tata Institute of Fundamental Research used laboratory experiments on rats to determine if abnormalities in the function of the amygdala help account for such a generalized fear reaction. Among other things, this brain structure helps all human beings stay grounded in reality by creating a specific context for their emotional experiences.

PTSD and Fear

In many cases, the appearance of fear is associated with activation of the “fight or flight” response, a survival reaction that prepares us for confrontation or a rapid retreat from danger. Normally, this response only kicks in for fairly brief amounts of time, then returns to its “off” position when a given threat fades away. However, in a person exposed to dangerous or possibly lethal events, the fight-or-flight response can fail to turn off and remain stuck in the “on” position. Doctors view the presence of this phenomenon, known technically as hypervigilance or hyperarousal, as one of the hallmark symptoms of PTSD.

In addition to fear in apparently harmless situations, potential indications of hyperarousal include unusual jumpiness, a tendency to startle easily and outbursts of anger or irritation that seem to occur for little or no reason. Fear can also appear in the context of two other common PTSD symptoms: the unwanted reliving of a previous traumatic event and a pressing desire to stay away from all circumstances that serve as a reminder of prior trauma exposure.

The Amygdala

The amygdala is a paired structure found in both halves of the human brain. It plays a central role in your emotional life by allowing you to process fear and other strong emotions. The amygdala also helps you put your emotional experiences in context and learn when to turn one emotion “on” and another “off.” In addition, the structure helps all human beings prioritize their reactions to specific situations. For instance, when you’re hungry, the amygdala helps you interpret your need to go and find food or eat any food you have on hand. When you face danger, the amygdala helps activate your fight-or-flight response.

PTSD, the Amygdala and Generalized Fear

In the study published in Nature Neuroscience, researchers from the Tata Institute of Fundamental Research used a combination of laboratory experiments and brain examinations to help determine the role that the amygdala plays in the development of generalized or indiscriminate fear in people dealing with PTSD. The researchers began by exposing a group of rats to a distinctive tone quickly followed by an electric shock. They also exposed the same group of animals to another distinctive sound that was not followed by a shock. Using brain examinations, the researchers gauged the amount of fear generated in the rats by each sound. They found that the animals’ fear levels spiked when they heard the sound associated with the electric shock but did not go up when they heard the sound not associated with the shock.

When they looked at the amygdala in each of the rats, the researchers found that the vast majority of the cells inside this structure increased their activity when the animals heard the shock-related sound. However, a small number of the cells increased their activity when the animals heard either sound. Next, the researchers upped the intensity of the electric shock administered to the animals. They found that this more extreme level of shock exposure made it much more likely that the cells inside the amygdala would increase their rate of activity when the rats heard either the shock-related tone or the non-shock-related tone.

Based on the results of their experiments and brain assessments, the researchers concluded that types of trauma capable of triggering PTSD may essentially “jolt” the cells inside the amygdala and make this brain structure lose its ability to put fear-inducing experiences in their appropriate contexts. In turn, this loss of context may explain why people affected by PTSD experience indiscriminate or generalized fear.

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