An experimental study exploring the impact of stress on memory revealed that participants demonstrated better recall of individual events when under stress compared to non-stressful conditions. However, their ability to recall the chronological sequence of events memorized during stress was inferior to that of events memorized in non-stressful situations. The study was published in eNeuro.
Stress encompasses both physiological and psychological responses to challenging or threatening scenarios, known as stressors. These stressors necessitate adaptation or change from the individual. The perception of a stressor triggers the body’s stress response system, leading to the release of hormones like cortisol and adrenaline, which prime the body for a “fight or flight” response. Stressors can originate externally, such as from work pressures or life events, or internally, like self-imposed expectations or health concerns.
Beyond physiological alterations, stress also induces numerous psychological changes, significantly impacting memory. Traumatic memories, which are forged under conditions of extreme stress, are usually exceptionally vivid and prone to involuntary recall, yet they tend to be fragmented and disjointed. Researchers associate these memory alterations with the effects of stress-induced noradrenaline and glucocorticoids on the prefrontal and medial temporal lobe regions of the brain.
The authors of the new study aimed to better understand how stress impacts memory formation. “In PTSD, memories of traumatic events are both exceptionally strong and vivid, yet fragmented and disintegrated. We were intrigued by this phenomenon and explored the possibility that acute stress could enhance the memory for individual elements of a stressful episode while diminishing the connections between these elements,” explained study author Lars Schwabe, the Head of Cognitive Psychology at the University of Hamburg.
The researchers posited that stress might promote a memory formation mode that enhances recall of individual events but hinders the processing of associations between them. To probe the brain mechanisms underlying these effects, the authors utilized functional near-infrared spectroscopy, focusing on the dorsolateral prefrontal cortex (dlPFC) and the inferior temporal gyrus (ITG), which are brain areas believed to be differentially influenced by stress.
The study involved 126 healthy volunteers, aged between 18 and 35 years, each of whom received 40 EUR for their participation. The researchers randomly assigned participants to two groups: one underwent a stress-inducing experimental procedure, while the other functioned as a control group.
The study spanned two days. On the first day, upon arrival at the lab, participants completed assessments for depression and anxiety symptoms using the State-Trait Anxiety Inventory and the Beck Depression Inventory, as well as for chronic stress using the Trier Inventory of Chronic Stress. They provided subjective stress ratings and saliva samples for cortisol measurement, a hormone released under stress. The researchers then affixed electrodes to the lower legs of participants in both groups, adjusting the electric shock intensity to be highly unpleasant yet non-painful, before later removing the electrodes.
During the experiment’s first phase, researchers presented participants with a series of pictures depicting outdoor environments. Participants were asked to determine whether the locations in the pictures were in the northern or southern hemisphere of Earth. The researchers refrained from providing feedback on the correctness of their responses.
In the experiment’s second block, the control group proceeded as before. Meanwhile, participants in the stress group were informed they would receive an electric shock if their answers were incorrect. Electrodes were placed on their legs, programmed to deliver 15 shocks of 200 milliseconds each, approximately 2.5-3 seconds after a picture was displayed. Unbeknownst to the participants, the shocks were administered regardless of the correctness of their answers. Following this 2.5-minute trial block, the electrodes were removed, and eight additional blocks followed, mirroring the procedure of the first block.
Throughout these tasks, the researchers recorded cortical activation in the participants’ brains using functional near-infrared spectroscopy and monitored various physiological parameters, including autonomic arousal, blood pressure, electrodermal activity, and heart rate. Functional near-infrared spectroscopy, a non-invasive neuroimaging technique, measures changes in blood oxygen levels in the brain by detecting near-infrared light absorption, offering insights into neural activity during cognitive tasks and other mental processes.
On the second day, participants were seated in a different room to negate the influence of environmental cues on memory recall (i.e., context-dependent memory effects). They repeated assessments of anxiety and stress, provided saliva samples, and reported their sleep quality and duration from the previous night. Researchers then showed them a series of pictures, including 360 from the first day and 180 new ones. Participants were asked to rate on a scale from 1 to 4 whether they recognized each picture from the previous day and their confidence level in their response.
Subsequently, they completed a sequence probing test, where they were shown two pictures from the previous day side by side and asked to determine if they were presented in the same block the day before.
The results confirmed that electric shocks effectively induced stress. The stress group reported higher subjective stress levels after the second trial 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 when they received shocks, indicating stress. Concurrently, activity in the dorsolateral prefrontal cortex decreased significantly.
Upon examining the recall of pictures from the previous day, the researchers found that the stress group remembered pictures from the block during which they received shocks significantly better than from the initial block without shocks. This difference was not observed in the control group, suggesting that stress enhanced the memorization of individual pictures.
However, the stress group struggled more with recalling the sequence of pictures in the block where they received shocks compared to the stress-free blocks. Further analysis revealed that stress altered memory functioning only in the presence of stressors (electric shocks). The recall of picture blocks shown after the electric shock block was comparable to the recall from the first block.
“The take-home message is as follows: while memory for individual elements of a stressful episode can be enhanced, this enhancement might come at the cost of reduced memory of how these elements relate to one another,” Schwabe told PsyPost. “Thus, stress appears to enhance certain aspects of memory but, simultaneously, impairs other facets of memory.”
The study sheds light on changes in memory functioning under stress. However, it should be noted that the stressor used in the study was very mild and of short duration. Results might not be the same on people exposed to more extreme stressors and for longer time periods.
“We used fNIRS, which can only measure lateral cortical activity but not activity in medial cortical or subcortical brain areas,” Schwabe noted. “Thus, in order to learn more about the involved brain mechanisms, future studies could use fMRI which allows the investigation of the entire brain.”
The paper, “Strong but Fragmented Memory of a Stressful Episode”, was authored by Anna-Maria Grob, Denise Ehlers, and Lars Schwabe.