Our memories are not stored in isolation. Rather, they form a dynamic network that can be constantly updated with new information, making us capable of learning. Imagine watching a suspenseful movie, full of twists and turns, in which you wonder who the wanted criminal might be. In the last scene, the grand finale, you discover who was the culprit all along, and in an “aha moment” you immediately connect scenes that were not connected for you before.
When we gain insight into the relationship between initially unrelated events, we integrate previously separate memory representations into coherent episodes – as we connect the dots by discovering the culprit in a movie. The hippocampus is central to this process of memory integration. Memory integration is, however, also a process that undergoes change in stress-related disorders: in post-traumatic stress disorder (PTSD), for example, memories appear fragmented. It has been well established that acute stress has a major impact on learning and memory and that these effects are mediated by glucocorticoids (mainly cortisol in humans) on prefrontal and medial temporal regions.
Therefore, we set out to investigate the effects of acute stress on memory integration by combining fMRI, neuroendocrinology, and representational similarity analysis with a life-like narrative-insight task. One week after encoding, we performed a comprehensive behavioral analysis of correctness, detailedness as well as memory representation. Our results showed that acute stress came with significant changes in the neural integration of initially separate events into coherent episodes. We found that stress reduced medial temporal lobe activity as participants gained insight into the relationship between events and, most importantly, prevented an insight-driven representational pattern change in the anterior hippocampus. Although stress reduced the neural changes associated with insight, it enhanced long-term memory, most likely due to the facilitating effect of glucocorticoids on memory. If we better understand the effects of stress on memory integration, it may also help us understand the disintegrated, fragmented memories in stress-related disorders, such as PTSD.