Memories of stressful events are stronger, but less integrated

Memories of stressful events are stronger, but less integrated

An experimental study exploring the effect of stress on memory revealed that participants showed better recall of individual events when under stress compared to non-stressful conditions. However, their ability to recall the temporal sequence of memorized events during stress was inferior to that of memorized events in non-stressful situations. The study was published in eNeuro.

Stress includes physiological and psychological responses to challenging or threatening scenarios, known as stressors. These stressors require adaptation or change from the individual. Perception of a stressor stimulates the body’s stress response system, resulting in the release of hormones such as cortisol and adrenaline, which prime the body for a “fight or flight” response. Stressors can arise externally, such as work pressures or life events, or internally, such as self-imposed expectations or health concerns.

Apart from physiological changes, stress also leads to many psychological changes, which greatly affects memory. Traumatic memories, formed under conditions of extreme stress, are usually exceptionally vivid and susceptible to involuntary recollection, yet they tend to be fragmentary and disjointed. Researchers link these changes in memory to the stress-induced effects of noradrenaline and glucocorticoids on the frontal and medial temporal lobe regions of the brain.

The authors of the new study aim to better understand how stress affects memory formation. “In PTSD, memories of traumatic events are exceptionally strong and vivid, but fragmented and disintegrating. This phenomenon sparked our interest and we explored the possibility that acute stress could enhance memory for elements,” explained study author Lars Schwabe, head of the Department of Cognitive Psychology at the University of Hamburg. Individuality in a stressful episode while reducing the connections between these elements.

The researchers hypothesized that stress may promote a mode of memory formation that enhances the recollection of individual events but hinders the processing of associations between them. To explore the brain mechanisms underlying these effects, the researchers used functional near-infrared spectroscopy, focusing on the dorsolateral prefrontal cortex (dlPFC) and inferior temporal gyrus (ITG), regions of the brain thought to be differentially affected by stress.

The study involved 126 healthy volunteers, aged between 18 and 35, who each received €40 for their participation. The researchers randomly divided the participants into two groups: one underwent the stress-inducing experimental procedures, while the other served as a control group.

The study extended for two days. On day 1, upon arrival at the laboratory, participants completed assessments of depressive and anxiety symptoms using the Trait Anxiety Inventory and the Beck Depression Inventory, as well as chronic stress using the Trier Chronic Stress Inventory. They provided self-assessments of stress and saliva samples to measure cortisol, a hormone released under stress. The researchers then attached electrodes to the lower legs of participants in both groups, adjusting the intensity of the electric shock to be extremely unpleasant but not painful, before later removing the electrodes.

During the first phase of the experiment, the researchers presented participants with a series of images depicting outdoor environments. Participants were asked to indicate whether the locations in the images were located in the Earth’s northern or southern hemisphere. The researchers declined to provide feedback on the validity of their answers.

In the second set of the experiment, the control group continued their work as before. Meanwhile, participants in the stress group were told that they would receive an electric shock if their answers were incorrect. Electrodes were placed on their legs and programmed to deliver 15 shocks, each lasting 200 ms, approximately 2.5 to 3 seconds after the image was presented. Without the participants’ knowledge, the shocks were administered regardless of the correctness of their answers. After this 2.5-min trial, the electrodes were removed, and eight additional blocks followed, mirroring the procedure of the first block.

During these tasks, the researchers recorded cortical activation in the participants’ brains using functional near-infrared spectroscopy and monitored several physiological parameters, including autonomic arousal, blood pressure, electrical activity, and heart rate. Functional near-infrared spectroscopy, a non-invasive neuroimaging technique, measures changes in blood oxygen levels in the brain by detecting the absorption of near-infrared light, providing insight into neural activity during cognitive tasks and other mental processes.

On the second day, participants sat in a different room to neutralize the influence of environmental cues on memory retrieval (i.e., context-dependent memory effects). They repeated anxiety and stress assessments, provided saliva samples, and reported on the quality and duration of their sleep from the previous night. The researchers then showed them a series of images, including 360 images from the first day and 180 new images. Participants were asked to rate on a scale of 1 to 4 whether they recognized each picture from the previous day and their level of confidence in their response.

Next, they completed a sequence scanning test, in which they were shown two pictures from the previous day side by side and were asked to indicate whether they had been shown in the same set the day before.

The results confirmed that electric shocks effectively cause stress. The stress group reported higher levels of personal stress after the second experimental block (during which they received electric shocks) compared to the control group. Functional near-infrared spectroscopy indicated increased activity in the inferior temporal gyrus of the stress group during the phase in which they received the shocks, indicating stress. At the same time, activity in the dorsolateral prefrontal cortex decreased significantly.

When examining recall of images from the previous day, the researchers found that the stress group recalled images from the block during which they received shocks significantly better than the initial group without shocks. This difference was not observed in the control group, suggesting that stress enhanced memorization of individual images.

However, the stress group had more difficulty remembering the sequence of pictures in the block in which they received the shocks than the stress-free groups. Further analysis revealed that stress alters memory performance only in the presence of stressors (electric shocks). Retrieval of the picture blocks shown after the electric shock block was similar to retrieval from the first block.

“The take-home message is this: While it is possible to enhance memory for the individual elements of a stressful episode, this improvement may come at the cost of decreased memory for how these elements relate to each other,” Schwabe told PsyPost. “Thus, stress appears to enhance certain aspects of memory, but at the same time, it impairs other aspects of memory.”

The study sheds light on changes in memory performance under stress. However, it should be noted that the stressors used in the study were very mild and of short duration. Results may not be the same in people exposed to more intense stressors for longer periods of time.

“We used fNIRS, which can only measure lateral cortical activity but not activity in medial cortical or subcortical brain regions,” Schwabe noted. “Thus, in order to learn more about the brain mechanisms involved, future studies could use functional MRI that allows the entire brain to be examined.”

The paper, “Strong but Fragmented Memory for a Stressful Episode,” is authored by Anna Maria Grubb, Dennis Ehlers, and Lars Schwabe.

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