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Memory, the Amygdala, and PTSD.

The fine line between remembering too much and too little.

What would it be like if we could remember everything that happens to us? Our lives would probably lose direction and meaning as was the case with S., the subject in A.R. Luria's classic case history, The Mind of the Mmenonist. Mr. S. could recall details of all his experiences even years after their occurrence. Yet, he never focused on a particular goal or lasted long at a job or career. Instead, he ended up performing as a mnemonist, astonishing crowds with his powers of recall. However, the details he recollected were often meaningless lists of words. Significant and trivial experiences were remembered in equal measure.

A few weeks ago, I visited a friend who lives in Manhattan. Her apartment windows overlook the site of the former World Trade Center, and she described to me her experience of watching the towers come down. "I can see it and smell it like it was yesterday," she said. That she remembered these events in such detail is not surprising to most of us. Emotionally charged events often produce our most powerful and intrusive memories.

Why do we remember frightening events so well? Fearful situations stimulate the brain to activate the sympathetic nervous system and adrenal glands causing the release of stress neurotransmitters and hormones. These chemicals activate the "flight or fight" response which includes an increase in heart rate to facilitate the delivery of blood to working muscles. They also stimulate a brain structure called the amygdala.

As mentioned in my last post, the amygdala is involved with our experience of fear. I described a woman, identified as S.M., who lacked a functioning amygdala and was indiscriminately trusting and friendly. A second individual, identified as B.P., who also lacks a functioning amygdala, was tested for his ability to remember frightening events. In one study, B.P. and control subjects were presented with a story that contained both emotionally neutral and disturbing events. Like the control subjects, B.P. reacted strongly to the emotionally arousing parts of the story right after it was told. One week after hearing the story, however, control subjects recalled the disturbing parts of the narrative better than the emotionally neutral parts. B.P. remembered the emotionally neutral sections as well as the control subjects, but he demonstrated no enhanced retention of the emotionally arousing parts. One function of the amygdala may be to assign significance to experiences, particularly those that are frightening and life-threatening, and then to enhance the ability of other brain regions to consolidate memories of those events.

The selective nature of our memory makes sense; our very survival may depend upon learning the lessons from life-threatening episodes. However, our ability to selectively remember traumatic episodes can also harm us. Seven percent of the general population suffer from Post-Traumatic Stress Disorder (PTSD). Episodes which produce intense fear can lead to PTSD with effects that may last for a month or persist for a lifetime Situations that remind the individual of the traumatic event or events can trigger excessive release of stress hormones and over-activation of the amygdala which further augments stress hormone release. The result is severe emotional distress - racing thoughts, anger, and hyper-vigilance. When it comes to the most traumatic events in our lives, we walk a fine line between remembering too much and remembering too little.

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