Perceived threat bias and reduced hippocampal volume in combat veterans

Reduced hippocampal volume is frequently observed in posttraumatic stress disorder (PTSD), but the exact psychological processes associated with these alterations remain unclear. Given the role of the hippocampus in contextual representations of threat and memory, we investigated relationships between retrospectively reported combat exposure, perceived threat, and hippocampal volume in trauma-exposed veterans. T1-weighted anatomical images were obtained from a sample of 52 male veterans with a broad range of PTSD symptoms during MRI scanning. Hippocampal volume was estimated using automatic segmentation tools in FreeSurfer. An index of Perceived Threat Bias (PTB) was calculated, reflecting the degree of discordance between subjective perceptions of threat while deployed and self-reported combat exposure. Hippocampal volume was regressed on PTB and PTSD symptoms on the Clinician-Administered PTSD Scale (CAPS). Results indicated that perceived threat bias was unrelated to overall CAPS symptoms, but was positively associated with CAPS avoidance/numbing symptoms and symptoms of depression, anxiety, and worry. Elevated PTB was associated with smaller hippocampal volume for individuals with a wide range of PTSD symptoms. Hippocampal volume was also inversely related to avoidance/numbing CAPS symptoms. These results indicate that volume of the hippocampus, a region involved in contextual threat processing and memory, is related to recalled associations between traumatic events and accompanying subjective threat appraisals. Future research should clarify the precise temporal milieu of these effects and investigate whether individual differences in hippocampal structure and function contribute to exaggerated threat appraisal at the time of trauma, or in subsequently biased memories or appraisals of traumatic events.