Weak and strong memories are equally reactivated during counterfactual learning, but only weak memories are modified

Adaptively updating memories about familiar others is an important social skill. Here, we examined whether strong or weak social memories are more prone to updating when violated. Participants viewed well-known characters’ emotional reactions to food items either repeatedly or only once. They then studied new events where some characters exhibited unexpected emotions to the same food items, violating initial memories. We found that when violated, strong existing memories, compared to weak memories, led to worse learning of new information, suggesting the resistance of strong memories to updating. Further, using a novel behavioral metric, we assessed the mechanistic role of memory reactivation in the updating of strong and weak memories. We showed that when encountering conflicting information, instead of reflecting their relative encoding strength, reactivation of weak memories reached a level similar to that of strong memories. Meanwhile, strong memories that were more reactivated showed higher likelihood to incorporate new information.