Strong but fragmented memory of a stressful episode.

While it is commonly assumed that stressful events are vividly remembered, it remains largely unknown whether all aspects of memory for a stressful episode are enhanced. In this preregistered study, we tested whether stress enhances later remembering of individual elements of a stressful episode at the cost of impaired processing of the association between these elements. Therefore, male and female participants (N=122) underwent a stressful (or control) episode during which they encoded a series of stimuli. To investigate stress effects on the memory for individual events and the links between these, we utilized temporal sequence effects in recognition memory tested 24 hours after encoding. Specifically, we tested whether stress would affect the memory enhancement for a target item if this is preceded by another item that also preceded the target during encoding (recognition priming). Our results showed that participants recalled single events encoded under stress better than those encoded under non-stressful conditions, but were less able to leverage the temporal sequence of events encoded under stress to cue memory at delayed recall, reflected in reduced memory for items preceded by the item that preceded them also during encoding. Functional near-infrared spectroscopy further revealed that encoding under stress was accompanied by opposite changes in infero-temporal and dorsolateral prefrontal areas. Together, our data suggest that acute stress induces a mode of memory formation that results in strong but less integrated memories. Significance Statement Stress has a major impact on memory, with critical implications for stress-related mental disorders such as posttraumatic stress disorder (PTSD). For trauma memory in PTSD, however, there is a remarkable contradiction: while memory for the trauma is typically so strong that it leads to involuntary recall, trauma memory is often fragmented and disintegrated. We hypothesized that these memory distortions might be due to a mode of memory formation under stress that enhances memory for individual events but impairs the processing of associations between these. In line with our hypothesis, we found that stress resulted in strong but fragmented memories. Stronger but less integrated memory was accompanied specific changes in frontal and temporal brain areas.