Our findings suggest that, following trauma exposure, carriers of two FKBP5 risk alleles may be more likely to develop decrements in this tract, and these decrements appear to influence functional connectivity. It is likely that these functional alterations, in turn, influence the development of the psychological and cognitive manifestations that are characteristic of PTSD. The cingulum is a major association fiber bundle that extends from the entorhinal cortex to the anterior cingulate cortex, connecting the parahippocampal gyrus, aspects of the parietal lobe, supplementary motor areas, medial and dorsolateral aspects of the prefrontal cortex (49). Given that it connects these areas, it is likely that abnormalities in cingulum connectivity adversely affect a number of relevant cognitive and emotional processes. One process is extinction of learned fear, a type of learning that is thought to be impaired in PTSD, and which is thought to rely on the quality of connections between medial prefrontal and limbic (i.e., amygdala and hippocampal) regions (50). In fact, emerging evidence suggests that cingulum connectivity is inversely associated with the magnitude of fear-potentiated startle response during extinction learning
