Preliminary evidence that stress makes negative memories less distinctive, with implications for witness testimony
Studies on stress and memory have often given conflicting results.
Stress has complicated effects on our memories.
Whereas some studies have found that we are better at remembering events that occurred during stressful situations, such as while watching disturbing videos, others have shown that stress impairs memory. Now a study published in Brain and Cognitionsuggests that stress doesn't influence the strength of our emotional memories at all. Instead, the researchers claim, it is the fidelity of those memories – how distinct and precise they are – that changes when we go through stressful experiences.
Maheen Shermohammeda from Harvard University and colleagues recruited 56 young adults between 18 and 23, and asked them to view a series of negative and neutral pictures. That might seem painless enough – except that half of the participants looked at the pictures while feeling rather stressed. Before they began looking at the pictures, they were told that they would later have to give a speech to a panel of judges. To make matters worse, before seeing each block of pictures they had to complete complicated maths problems. They were given just a short time to complete these, and while doing so they were told that they were performing poorly and their data would be unusable if they didn't do well. In contrast, the control group had a fairly relaxing time: instead of a speech, they were told they would have to write a story, and they only had to complete simple maths problems at their own pace.
About two weeks later, all the participants were given a surprise memory task, in which they again saw the earlier pictures alongside new images they hadn't seen before. They had to indicate whether each picture was an old one that had been in the original task or a new image they hadn't seen before.
At several points throughout the study, all participants were asked how stressed they were, and also had their heart rate measured and saliva samples taken to analyse levels of the stress hormone cortisol. As expected, the group that went through the stressful experience reported higher levels of stress, and had increased heart rates and cortisol levels (although the team had to exclude a handful of participants who, surprisingly, didn't report feeling stressed).
Overall, participants in both groups were better at remembering negative images – this replicates a well-established finding that emotional material tends to be more memorable. Also, the stressed group correctly remembered just as many of the earlier images as the control group (i.e. their "hit rate" was the same). Crucially, where the groups differed was in their patterns of "false alarms" – how often they falsely remembered new images as being from the original task. The stressed participants were more prone to false alarms for negative images compared neutral images, and the more stressed they were, the larger this difference.The control group didn't show any difference between the two kinds of images.
Based on these results, the researchers suggest it's not the strength of our memories that is influenced by stress, but rather their fidelity, or how distinct they are from other information. For the stressed group, neutral memories became more distinct, making it easier to distinguish them from new neutral material, while negative memories were more vague or blurred, making it harder to distinguish them from new negative material. The results demonstrate the importance of separating out memory into its constituent parts (strength and fidelity), the researchers add, rather than just looking at overall performance or correct "hits".
It's clearly a rather preliminary result. The pool of participants was small to begin with, and made even smaller after the researchers had to remove those participants who didn't respond to their stress intervention. And the participants all came from a very young, narrow age range, raising the question of whether older people respond in similar ways.
Nevertheless, the idea that stress has different effects on different components of memory is an interesting proposition that deserves further attention – particularly as understanding memory during periods of stress has important real-world implications for situations like eyewitness accounts of crimes. For example, the researchers said, a "stressed witness to [a] crime … may indeed have a strong recollection of the criminal, but may also have an impoverished ability to discriminate the assailant from other individuals in a police lineup."
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A trio of intriguing galaxy clusters<img type="lazy-image" data-runner-src="https://assets.rebelmouse.io/eyJhbGciOiJIUzI1NiIsInR5cCI6IkpXVCJ9.eyJpbWFnZSI6Imh0dHBzOi8vYXNzZXRzLnJibC5tcy8yNDQzNDA0OS9vcmlnaW4uanBnIiwiZXhwaXJlc19hdCI6MTYxNTkzNzUyOH0.0IRzkzvKsmPEHV-v1dqM1JIPhgE2W-UHx0COuB0qQnA/img.jpg?width=980" id="d69be" class="rm-shortcode" data-rm-shortcode-id="2d2664d9174369e0a06540cb3a3a9079" data-rm-shortcode-name="rebelmouse-image" />
The three galaxy clusters imaged for the study
Mapping dark matter<span style="display:block;position:relative;padding-top:56.25%;" class="rm-shortcode" data-rm-shortcode-id="d904b585c806752f261e1215014691a6"><iframe type="lazy-iframe" data-runner-src="https://www.youtube.com/embed/fO0jO_a9uLA?rel=0" width="100%" height="auto" frameborder="0" scrolling="no" style="position:absolute;top:0;left:0;width:100%;height:100%;"></iframe></span><p>The assumption has been that the greater the lensing effect, the higher the concentration of dark matter.</p><p>As scientists analyzed the clusters' large-scale lensing — the massive arc and elongation visual effects produced by dark matter — they noticed areas of smaller-scale lensing within that larger distortion. The scientists interpret these as concentrations of dark matter within individual galaxies inside the clusters.</p><p>The researchers used spectrographic data from the VLT to determine the mass of these smaller lenses. <a href="https://www.oas.inaf.it/en/user/pietro.bergamini/" target="_blank" rel="noopener noreferrer">Pietro Bergamini</a> of the INAF-Observatory of Astrophysics and Space Science in Bologna, Italy explains, "The speed of the stars gave us an estimate of each individual galaxy's mass, including the amount of dark matter." The leader of the spectrographic aspect of the study was <a href="http://docente.unife.it/docenti-en/piero.rosati1/curriculum?set_language=en" target="_blank">Piero Rosati</a> of the Università degli Studi di Ferrara, Italy who recalls, "the data from Hubble and the VLT provided excellent synergy. We were able to associate the galaxies with each cluster and estimate their distances." </p><p>This work allowed the team to develop a thoroughly calibrated, high-resolution map of dark matter concentrations throughout the three clusters.</p>
But the models say...<p>However, when the researchers compared their map to the concentrations of dark matter computer models predicted for galaxies bearing the same general characteristics, something was <em>way</em> off. Some small-scale areas of the map had 10 times the amount of lensing — and presumably 10 times the amount of dark matter — than the model predicted.</p><p>"The results of these analyses further demonstrate how observations and numerical simulations go hand in hand," notes one team member, <a href="https://nena12276.wixsite.com/elenarasia" target="_blank">Elena Rasia</a> of the INAF-Astronomical Observatory of Trieste, Italy. Another, <a href="http://adlibitum.oats.inaf.it/borgani/" target="_blank" rel="noopener noreferrer">Stefano Borgani</a> of the Università degli Studi di Trieste, Italy, adds that "with advanced cosmological simulations, we can match the quality of observations analyzed in our paper, permitting detailed comparisons like never before."</p><p>"We have done a lot of testing of the data in this study," Meneghetti says, "and we are sure that this mismatch indicates that some physical ingredient is missing either from the simulations or from our understanding of the nature of dark matter." <a href="https://physics.yale.edu/people/priyamvada-natarajan" target="_blank">Priyamvada Natarajan</a> of Yale University in Connecticut agrees: "There's a feature of the real Universe that we are simply not capturing in our current theoretical models."</p><p>Given that any theory in science lasts only until a better one comes along, Natarajan views the discrepancy as an opportunity, saying, "this could signal a gap in our current understanding of the nature of dark matter and its properties, as these exquisite data have permitted us to probe the detailed distribution of dark matter on the smallest scales."</p><p>At this point, it's unclear exactly what the conflict signifies. Do these smaller areas have unexpectedly high concentrations of dark matter? Or can dark matter, under certain currently unknown conditions, produce a tenfold increase in lensing beyond what we've been expecting, breaking the assumption that more lensing means more dark matter?</p><p>Obviously, the scientific community has barely begun to understand this mystery.</p>
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