It is hardly surprising that many (though not all) neuroimaging studies of PTSD find increased amygdala activation in response to traumatic reminders. A large body of animal research, mostly in rodents, has established the importance of the amygdala for emotional processes (Cahill and McGaugh, 1998; LeDoux, 1996). The amygdala establishes the initial interpretation of the nature of a particular stress and initiates the process of activating neurochemical and neuroanatomical fear circuitries (LeDoux, 1992). The time frame for this response is several milliseconds. In this very short time, projections from the amygdala to the reticularis pontis caudalis potentiate the startle responses and initiate defensive behaviors that do not require direct action of the sympathetic nervous system. Projections from the amygdala to the lateral hypothalamus and then to the rostral ventral medulla initiate sympathetic nervous system (and catecholamine) responses. One of the most immediate responses to stress is the coordinated sympathetic discharge that causes increases in heart rate and blood pressure, initially described by Walter Cannon as the fight-or-flight reaction. Exposure to traumatic reminders provokes autonomic activation in about two-thirds of patients with PTSD (e.g., Pitman et al., 1987), and this is likely mediated by activation of the amygdala and related structures.
Projections from the amygdala to the solitary tract initiate the parasympathetic responses that constrain autonomic arousal but operate independently of the sympathetic nervous system. Projections from the central amygdala to the bed nucleus of the stria terminalis initiate the HPA axis response. By way of these various connections, the amygdala transforms sensory stimuli into emotional and hormonal signals, thereby initiating and controlling emotional responses.
Numerous studies have reported activation of the amygdala during early phases of aversive conditions, showing that the amygdala is necessary for the establishment of conditioned fear (e.g., LaBar et al., 1998). Most of these studies have focused on fear perception, demonstrating that the amygdala is important for the recognition of cues of threat or danger. For example, the amygdala is activated in response to facial expressions of fear, compared with neutral, happy, or other control faces, even when people are exposed to masked-fear faces that were not consciously perceived (Whalen et al., 1998). Whether the amygdala is necessary for the expression of fear and whether the amygdala is the actual focus of where the learned information is stored are still unclear (see Packard and Cahill, 2001; Fanselow and LeDoux, 1999). Since Breiter et al. (1996) observed rapid habituation of the amygdala response, and since some neuroimaging studies of PTSD subjects fail to find amygdala activation during symptom provocation paradigms, it is likely that the amygdala has a time-limited function in the stream of affective information processing.
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