The proposed R21 project will attempt to further develop a novel intervention for posttraumatic stress symptoms inspired by the science of memory reconsolidation. Work in normal humans has shown that when a stable, consolidated memory is reactivated (i.e., retrieved) under appropriate conditions, it reverts to an unstable state, a process referred to herein as deconsolidation. In such a state, the memory is susceptible to the action of various amnestic agents that may inhibit its reconsolidation, thereby weakening it. The β-adrenergic blocker propranolol (PPNL) possesses such amnestic properties. More recent research has found that in order to initiate deconsolidation, there must be a prediction error, or mismatch, between what is expected and what occurs when the memory is reactivated. Prior placebo-controlled, randomized clinical trials (PBO-RCT) from our laboratory have found that when propranolol is administered concomitant with the reactivation of a psychologically traumatic memory, the memory is weakened, as revealed by subsequent lower physiological (heart rate, skin conductance, facial electromyogram) responding during script-driven mental imagery. Clinical applicability was evaluated in a PBO-RCT, in which PTSD participants receiving propranolol underwent six weekly sessions of 10-20 min of standard (STD) traumatic memory reactivation stimulated by reading a narrative. At post-treatment, these participants showed a greater reduction of PTSD symptoms compared to participants who had taken PBO. The goal of the proposed study is to test whether intentionally incorporating innovative mismatch (MM) into traumatic memory reactivation can improve upon physiological responding during script-driven mental imagery. Participants will be randomized to one of 2 treatment arms: STD/PPNL and MM/PPNL. A baseline assessment will measure psychophysiological responsivity to script-driven mental imagery (target measure). PPNL will be administered 90-min prior to each of six weekly 10-20 min. traumatic memory reactivation sessions. In the MM condition, a different, unexpected mismatch (e.g., singing the narrative) will be incorporated into the reactivation. In the STD condition, the participant will read the narrative the same way each time. The focus of the R21 proposal will be to assess whether the MM/PPNL group shows lower subsequent physiological responses than the STD/PPNL group