Background. Dissociative reactions in post-traumatic stress disorder (PTSD) have been regarded as strategic responses that limit arousal. Neuroimaging studies suggest distinct prefrontal responses in individuals displaying dissociative and hyperarousal responses to threat in PTSD. Increased prefrontal activity may reflect enhanced regulation of limbic arousal networks in dissociation. If dissociation is a higher-order regulatory response to threat, there may be differential responses to conscious and automatic processing of threat stimuli. This study addresses this question by examining the impact of dissociation on fear processing at different levels of awareness.Method. Functional magnetic resonance imaging (fMRI) with a 1.5-T scanner was used to examine activation to fearful (versus neutral) facial expressions during consciously attended and non-conscious (using backward masking) conditions in 23 individuals with PTSD. Activation in 11 individuals displaying non-dissociative reactions was compared to activation in 12 displaying dissociative reactions to consciously and non-consciously perceived fear stimuli.Results. Dissociative PTSD was associated with enhanced activation in the ventral prefrontal cortex for conscious fear, and in the bilateral amygdala, insula and left thalamus for non-conscious fear compared to non-dissociative PTSD. Comparatively reduced activation in the dissociative group was apparent in dorsomedial prefrontal regions for conscious fear faces.Conclusions. These findings confirm our hypotheses of enhanced prefrontal activity to conscious fear and enhanced activity in limbic networks to non-conscious fear in dissociative PTSD. This supports the theory that dissociation is a regulatory strategy invoked to cope with extreme arousal in PTSD, but this strategy appears to function only during conscious processing of threat. IntroductionPost-traumatic stress disorder (PTSD) is characterized by abnormal fear reactions, ranging from hyperreactivity to dissociative responses to threat. Dissociative phenomena occur in a substantial proportion of individuals with PTSD, and include emotional numbing, psychogenic amnesia, depersonalization and derealization symptoms. It is argued there may be a subtype of PTSD that involves an ongoing 'dissociative ' rather than 'hyperarousal ' reaction to threat (Bremner, 1999). In this context, dissociation has been conceptualized as an avoidant strategy to reduce awareness of aversive emotions such as extreme anxiety (Griffin et al. 1997).The neural mechanisms underlying dissociative responses in PTSD are insufficiently known. Neuroimaging studies suggest there may be distinctive prefrontal responses to trauma memories in individuals with dissociative and non-dissociative PTSD responses (Lanius et al. 2006). Individuals displaying a 'hyperarousal ' PTSD response to traumatic narratives, with heightened autonomic and emotional reactivity to trauma scripts, have reduced bilateral medial frontal activity [Brodmann area (BA) et al. 2002). Consistent with this propo...

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Although women have a greater propensity than men to develop posttraumatic stress disorder (PTSD) following trauma, sex differences in neural activations to threat have received little investigation. This study tested the prediction that trauma would heighten activity in automatic fear-processing networks to a greater extent in women than in men. Functional magnetic resonance imaging (fMRI) data were recorded in 23 participants with PTSD (13 women, 10 men), 21 trauma-exposed controls (9 women, 12 men), and 42 non-trauma-exposed controls (22 women, 20 men) while they viewed masked facial expressions of fear. Exposure to trauma was associated with enhanced brainstem activity to fear in women, regardless of the presence of PTSD, but in men, it was associated only with the development of PTSD. Men with PTSD displayed greater hippocampal activity to fear than did women. Both men and women with PTSD showed enhanced amygdala activity to fear relative to controls. The authors conclude that greater brainstem activation to threat stimuli may contribute to the greater prevalence of PTSD in women, and greater hippocampal activation in men may subserve an enhanced capacity for contextualizing fear-related stimuli.

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