Our knowledge and memories help us draw new conclusions in everyday life. If we see a friend’s car in front of a supermarket, we can assume that they are shopping there. However, when we are under stress, it is more difficult for us to make such associations, as a study now shows. Accordingly, when we are under acute stress, we are more likely to form individual memories rather than linking the new memory content with existing ones.
Our memory enables us to make connections between different impressions and memories and to gain new information from them. “If a friend shows you his new, light blue Vespa and you later see the same scooter standing in front of the university library, you could conclude that your friend is currently studying inside,” explains a team led by Kai Schüren from the University of Hamburg.
This so-called memory integration requires that we use our knowledge of the connection “friend – light blue vesper” to infer our friend from the new connection “light blue vesper – library”. “Brain regions that are important for this process, especially the hippocampus, have a particularly large number of receptors for stress messengers,” write the researchers. “However, it is unclear whether stress affects memory integration and, if so, what neural mechanisms underlie such stress-related changes.”
Memory test under stress
To answer this question, Schüren and his colleagues first had 121 volunteers learn connections between pairs of images. Picture A showed a face or a scene, picture B showed an animal. The next day, the team exposed half of the volunteers to acute stress by conducting a fictitious job interview with them and also giving them cognitively demanding tasks. Instead, the control group was allowed to talk about a topic of their own choosing, such as their last vacation. As expected, the participants who were put under stress subsequently had higher blood pressure and pulse and also had increased levels of the stress hormone cortisol in their saliva.
All volunteers then learned further connections between images, this time between the animal images already known from the previous day and a new category C with geometric shapes. For example, if the test subjects had seen a cat next to a woman’s face the day before, they were now presented with the same cat next to a stack of rectangles. After everyone had internalized the new connections, the actual test followed: Can the test subjects associate images A and C with each other? So can they match the stack of rectangles to the woman’s face because they saw both in connection with the cat?
Flexible associations restricted
The result: The people in the stressed group found it more difficult to link the AC pairs than the unstressed control group. These differences could not be explained by the fact that the stressed test subjects learned the AB or BC pairs more poorly. Because in memory tests aimed at this, they performed just as well as their colleagues from the control group. What they were missing was apparently just the connection. “Apparently the stress impaired the ability to connect related episodes,” the researchers report. “This led to deficits in associative reasoning.”
Using functional magnetic resonance imaging (fMRI), Schüren and his team made visible what was happening in the volunteers’ brains during learning and retrieval: When people from the control group saw the animal images from category B again on the second day of the experiment, their hippocampus reactivated the memory of the associated image A. This also automatically linked this image to the new image C. The stressed test subjects, on the other hand, had a narrower focus: their memories of image A were hardly reactivated and the new connection between images B and C was stored independently of the existing knowledge.
“Our results suggest that under stress the brain prioritizes the clear representation of individual episodes over the formation of networked knowledge structures,” conclude Schüren and his colleagues. From their point of view, the results are relevant in many areas of society. “Integrating related events in memory is crucial for building knowledge that goes beyond direct perception and allows for flexible inferences,” they write. “Our results show that acute stress impairs a key mechanism of memory integration, which has broad implications for the educational, legal and clinical areas.”
Source: Kai Schüren (University of Hamburg) et al., Science Advances, doi: 10.1126/sciadv.aea5496