Posttraumatic stress disorder is not an inevitable outcome of stress: Only about 25 percent of individuals who have been exposed to a potential traumatic stressor develop PTSD (Yehuda and McFarlane, 1995). Hence, the central question regarding the biology of PTSD is how to account for the failure of the organism to reestablish its homeostasis and return to its pretraumatic state. Yehuda (2002) has pointed out that understanding the biological response that occurred during the traumatic event does not necessarily address the biology of PTSD. Rather, the central issue appears to be why some people recover and others do not.
It also has become clear that PTSD is not an issue of simple conditioning. Many people who have been exposed to an extreme stressor, but who do not suffer from PTSD, become distressed when they are once again confronted with the memory of the tragedy. The critical issue in PTSD is that the stimuli that cause people to overreact may not be conditional enough: A variety of triggers not directly related to the traumatic experience may come to precipitate extreme reactions. (Pitman et al., 1991).
Abnormal psychophysiological reactions in PTSD occur on two very different levels: (1) in response to specific reminders of the trauma and (2) in response to intense, but neutral, stimuli, such as loud noises, signifying a loss of stimulus discrimination.
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