Striatal habit system drives behavioral rigidity in PTSD

Core symptoms of posttraumatic stress disorder (PTSD) like safety seeking and avoidance have long been described as habitual. Although widespread theoretical accounts posit a relationship between these behaviors and striatal-mediated habits, or rigid stimulus-response (S-R) associations, this idea has yet to be tested. Here, we implement a new task design to investigate how S-R associations are acquired, how flexibly they can be remapped, and how sensitive they are to changes in outcome value for individuals with PTSD as compared to individuals who experienced trauma without developing PTSD (trauma-exposed controls; TC). We find that individuals with PTSD, compared to TC, exhibit several hallmarks of habitual behavior, including more rigid habit-like perseveration following response remapping, and, with more severe symptoms, continued perseveration following outcome devaluation. We also observe that the PTSD group acquired these S-R associations more slowly. Using functional magnetic resonance imaging, we find significant putamen engagement when learning S-R associations that was especially pronounced in PTSD and predicted later perseveration. Finally, computational models indicate that the PTSD group leveraged gradual trial-by-trial updating to learn and later update associations, whereas the TC group leaned more on a stable, consistent choice strategy, both of which explained later behavioral rigidity. In addition to distinguishing individuals with PTSD, these habit-related processes were also more pronounced for individuals with more severe symptoms. Together, these results provide the first empirical evidence for the role of habits in PTSD.