Modulatory effects of stress on reactivated emotional memories.

Previous studies have shown that stress, through secretion of stress hormones, increases the consolidation of memory while it exerts negative effects on memory retrieval. Other studies show that the process of memory retrieval serves as a reactivation mechanism whereby the memory trace that is reactivated during the retrieval process is once again sensitive to modifications by pharmacological or environmental manipulations. In this study, we assessed whether exposure to stress after retrieval of neutral and emotional information modulates the immediate and long-term recall of these reactivated memory traces. Three groups of participants (total N of 47) encoded on Day 1 a movie containing neutral and emotional information. Two days later (Day 2), one group was asked to retrieve (reactivate) the story before being exposed to a stressful condition (reactivation/stress group), while the second group was asked to retrieve the story and was not exposed to a stressful condition (reactivation/no stress group). A third group did not recall the story but was exposed to a stressful condition (no reactivation/stress group). All participants were asked to recall the story immediately after exposure to the stress/no stress condition (immediate recall) as well as 5 days later (delayed recall). Results show that immediate recall of emotional information was significantly increased in the reactivation/stress group when compared to the reactivation/no stress group while no effect of stress on reactivated neutral memories was found. Moreover, evidence suggests that the enhanced memory trace is maintained across time, suggesting a potential long-lasting effect of stress on reactivated memory traces. We also found that the enhanced emotional memory trace observed in the reactivation/stress group was not present in the no reactivation/stress group, showing that stress has the capacity to enhance memory only when the memory trace is acutely reactivated before exposure to stress. Altogether, these results suggest that stress differentially modulates reactivated emotional and neutral memory traces and that this effect is long-lasting. These results have important implications for the potential influence of acute stress on reactivated memories in individuals exposed to traumatic events.