People with posttraumatic stress disorder (PTSD) report heightened physiological responses to internal or external cues that symbolize or resemble an aspect of the trauma (1). Clinical observations suggest that these responses can be triggered by a wide range of stimuli. Furthermore, clinical observations suggest that people with PTSD may react with emotional and physiological arousal to situations that have little similarity with the trauma but overlap in particular sensory impressions (e.g. a similar color, smell or sound; 2). Models of PTSD draw on associative learning mechanisms to explain this heightened physiological and emotional reactivity (2-6). It has been suggested that during trauma fear responses become associated with stimuli that are present at the time, and subsequently generalize more broadly to stimuli and situations that resemble the original trauma (4 - 5). One way of investigating the role of associative learning in PTSD is to measure physiological responses to trauma reminders. Several studies have demonstrated a heightened physiological reactivity in trauma survivors with PTSD compared to traumatized and non-traumatized controls (for reviews see 7 - 9). This is consistent with the notion that learned fear responses acquired during trauma contribute to PTSD. There has been a longstanding interest in trauma research whether psychophysiological responses are useful in diagnosing PTSD and in identifying people at risk of chronic PTSD soon after trauma (7). Psychophysiological measures may provide useful additional information to the widely used self-report measures. In Pole’s meta-analysis (9), psychophysiological responses to standardized trauma cues identified PTSD with a mean sensitivity of .77 and a mean specificity of .91. Responses to idiographic trauma cues identified PTSD with a mean specificity of .65 and mean specificity of .83. However, most of the studies to date were cross-sectional comparisons of survivors of very distant traumas, such as combat or sexual abuse in childhood (e.g. 10, 11). This limits the conclusiveness of the results for the development of PTSD, as the heightened physiological reactivity may be a consequence of chronic PTSD rather than a factor contributing to its development. It is thus unclear whether the reasonable sensitivity and specificity observed in these studies also applies for the early identification of trauma survivors at risk of chronic PTSD. This question is of considerable interest as many survivors recover on their own (12) and it is unclear how best to identify those who need intervention. Few studies to date have investigated physiological responses to trauma reminders soon after trauma. Elsesser et al. (13) found that chronic PTSD patients and recent trauma survivors who met criteria for acute stress disorders (ASD) at 6 weeks after the trauma showed heart rate acceleration to individualized trauma-related pictures, whereas nontraumatized controls and survivors without ASD showed heart rate deceleration. The groups did not differ in heart rate responses (HRR) to generally threatening or neutral pictures. Blanchard et al. (14) studied survivors of motor vehicle accidents (MVA) at about 2.5 months after the trauma and found that HRR to audiotaped individualized scripts describing the participants’ accident, but not responses to other stressors, distinguished survivors with PTSD from those without PTSD and nontraumatized controls. These results are in line with the notion that in the initial months after trauma, PTSD is characterized by strong learned fear responses to reminders of the trauma. There is also preliminary evidence that HRR to trauma reminders can be used to predict the chronicity of PTSD. Elsesser et al. (15) followed up 35 recent trauma survivors from their 2004 study and found that greater HRR to the individualized trauma-related pictures predicted PTSD symptoms 3 months later. Blanchard et al. (14) found that HR acceleration to individualized MVA scripts predicted chronicity of PTSD at 1 year in 48 participants who had PTSD at the initial assessment. Kleim, Ehlers and Glucksman (16) found that HRR to guided imagery of the trauma at 2 weeks predicted PTSD severity at 6 months after the trauma in female, but not male assault survivors. These studies are of possible clinical relevance as they suggest that early psychophysiological responses may be useful in identifying trauma survivors at risk of chronic PTSD. So far, the studies that investigated whether early physiological responses to trauma reminders predict PTSD used idiographic trauma reminders that were selected because of their personal relevance to the participants, mainly script-driven-imagery of one’s trauma (14, 16) or idiographic pictures selected for their personal relevance to each participant (13). This raises the question of whether HRR to standardized sets of trauma reminders would also be predictive of chronic PTSD. If associative learning theories of PTSD are correct, then one would expect stimulus generalization soon after the trauma to contribute to the chronicity of PTSD. The present study was designed to investigate whether HRR to standardized trauma-related pictures can be used to identify people who will develop chronic PTSD. This question is of potential practical interest as many trauma survivors recover from initial symptoms of PTSD (12) and screening tools that allow the identification of those at risk of chronic PTSD are needed for the efficient allocation of scarce treatment resources. Currently, such screening relies on self-report symptom measures (17), as studies have shown that initial symptom severity is a relatively good predictor of chronic PTSD (18). It was therefore of particular interest to investigate whether HRR to standardized trauma pictures predicts chronic PTSD over and above what could be predicted from self-reported symptoms at one month after the trauma. The present prospective study assessed trauma survivors at 1 and 6 months. The study built on two earlier studies suggesting that recent trauma survivors with PTSD and patients with chronic PTSD may show heightened HRR to standardized trauma-related pictures compared to those without PTSD (20, 21). In contrast, however, Blanchard et al. (14, 19) found that HRR to guided imagery of a standard MVA script or videotapes of car crashes did not distinguish between survivors with and without PTSD. The present study also explored whether skin conductance responses to trauma reminders are related to PTSD. Previous studies have suggested that HRR are particularly sensitive in detecting response differences between people with and without PTSD (9). In studies of early responses after trauma, HRR also showed the most robust findings. In Elsesser et al.’s (13) study, startle responses did not show an interaction between group and picture type. Similarly, in Blanchard et al.’s (14) study, EMG and blood pressure responses did not distinguish between the PTSD and no PTSD groups. Skin conductance responses (SCR) were chosen for this study because SCR are widely used as a measure of conditioned emotional responses in laboratory studies (e.g. 22) and because SCR has been shown to differentiate between people with chronic PTSD and controls in some, but not all idiographic trauma cue studies (9). Furthermore, Elsesser et al.’s study (13) did not find any differences in SCR between participants with and without PTSD (unpublished data, personal communication, Dr. Karin Elsesser, September 18, 2009). Finally, the present study examined whether HRR to trauma reminders are related to the participants’ emotional and cognitive responses during the trauma, in particular to the degree of peri-traumatic fear and dissociation. These responses have been shown to predict PTSD (23- 25). The literature on conditioning suggests that stronger emotional arousal during trauma leads to stronger conditioned emotional responses (26, 27). Peri-traumatic dissociation is thought to decrease focal attention, and to promote a perceptual processing style that is characteristic of PTSD (28, 29). Ehlers, Hackmann and Michael (30) proposed that perceptual processing during trauma facilitates memory processes that rely on perceptual operations, such as associative learning. In sum, the present study investigated the following hypotheses: Hypothesis 1: Participants with PTSD show greater HRR to trauma-related pictures than participants without PTSD, but not to generally threatening or neutral pictures. Hypothesis 2: Greater HRR to trauma-related pictures at 1 month predict greater PTSD symptom severity at 6 months. Hypothesis 3: Peri-traumatic dissociation and fear during trauma predict greater HRR to trauma-related pictures at 1 month. Furthermore, the study explored whether SCR to standardized trauma cues is associated with PTSD as the previous findings on this response are mixed.