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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Going from a solitary teenage protester in front of the Swedish parliament to a global icon in little more than a year certainly merits the distinction.