Heterogeneity of defensive responses after exposure to trauma: blunted autonomic reactivity in response to startling sounds.

Research on threat responses, particularly among trauma-exposed individuals, has traditionally focused on increased autonomic arousal and reactivity. However, clinical features associated with trauma exposure, such as dissociation (e.g., shutting down or "spacing out") manifest as the opposite pattern: non-reactivity and blunted arousal. These clinical features suggest that the possibility of threat responses other than fight/flight, namely, immobilization may be undergirded by hyper- or hypo-arousal. The goal of this paper is to examine autonomic responses to a stressful stimulus (acoustic startle) using analytic approaches which have been previously used to examine defensive responses before: heart rate acceleration, heart rate deceleration, and skin conductance response. We examined these responses in relation to symptoms (Posttraumatic Stress Disorder, or PTSD, and dissociation) and trauma exposure (cumulative exposure, age of onset) in a sample of trauma-exposed college students. We found evidence of blunted reactivity, with decreased acceleration and skin conductance, but with increased deceleration, particularly among individuals who had significant symptoms and early exposure to multiple types of trauma. However, individuals with sub-clinical symptoms and more attenuated exposure had large heart rate acceleration and skin conductance responses during the task. Taken together, these findings suggest that moderate symptoms and trauma exposure are related to exaggerated autonomic responses, while extreme symptoms and trauma exposure are related to blunted autonomic responses. These findings further suggest heterogeneity of stress responses within individuals with PTSD and with trauma exposure.