Brain scans reveal how repeated exposure to emotional events shapes memory

Why do some memories, especially those tied to strong emotions, feel so much more vivid and persistent? A recent study published in The Journal of Neuroscience provides answers, revealing that repeated exposure to emotional events leads to the formation of exceptionally stable memory patterns in the brain. This process, initiated by the amygdala during the first encounter with the event, explains why emotional memories can be so powerful and long-lasting.

Emotional events tend to stick in our minds more vividly than everyday occurrences. This is generally a helpful ability, allowing us to learn from experiences that carry emotional weight. However, this strong emotional memory system can sometimes become problematic, particularly in conditions like posttraumatic stress disorder, where intrusive and distressing memories can be a hallmark of the disorder.

While scientists have learned a great deal about how we remember emotional events that happen just once, much less is understood about how our brains process and remember emotional events that we experience repeatedly. This is a significant gap in our knowledge because, in real life, many emotional experiences, whether positive or negative, tend to recur. To address this, researchers aimed to explore the brain mechanisms responsible for remembering repeated emotional events. They were particularly interested in testing two competing ideas about how repetition strengthens memory.

One idea suggests that each time we experience an event, it is encoded slightly differently in the brain due to changes in context or other factors, creating multiple pathways to access the memory. The other idea proposes that repetition reinforces the original memory trace, making it stronger each time it is reactivated. Previous studies focusing on neutral, non-emotional events have leaned towards the reinforcement idea, but it was unknown if this principle also applied to emotional memories.

“Most research so far examined memory for emotional events that were encountered just once and the brain mechanisms associated with the repeated exposure to the same emotional events were largely unknown,” explained study author Lars Schwabe, the Head of Cognitive Psychology at the University of Hamburg. “We wanted to investigate whether the repeated exposure to an emotional event is linked to more robust/similar neural representations or to more variable/specific representations and which mechanisms would drive the robust vs. variable representations.”

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To conduct their investigation, the research team recruited 103 healthy adults. These volunteers underwent brain scanning using magnetic resonance imaging while participating in a memory task. During the scan, participants viewed a series of images across three consecutive sessions. In each session, they were shown the exact same set of 30 emotionally negative images and 30 emotionally neutral images. The images, depicting a range of scenes, were selected from established image databases and online sources.

The researchers made sure that the negative and neutral images were comparable in terms of visual complexity and whether they featured people or animals. Within each session, the images were presented in a random order. Each image was displayed for three seconds, followed by a brief pause. Participants were instructed to pay close attention and try to memorize the images, as they were informed that their memory would be tested shortly after.

To ensure participants remained focused during the task, they were asked to press a button each time a fixation cross appeared on the screen. Immediately after the brain scanning session, participants completed a memory test outside of the scanner. They were given 15 minutes to verbally recall as many of the images as possible, describing them in detail. These verbal accounts were audio-recorded and subsequently evaluated by three independent raters who judged whether each recalled image was clearly identifiable from the participant’s description. The raters showed a very high level of agreement in their evaluations.

To validate that the images truly differed in emotional content, the participants, on a separate day, rated each image for how negative or positive and how emotionally arousing they found it. As expected, the negative images were rated as significantly more negative and emotionally arousing than the neutral images. The brain scan data collected during the image viewing task was then analyzed to examine both the overall level of brain activity and the patterns of activity in different brain regions known to be involved in emotion and memory, such as the amygdala, the hippocampus, and the neocortex.

The results of the study confirmed that participants remembered significantly more of the emotionally negative images compared to the neutral images, demonstrating the well-known emotional memory advantage. Analysis of the brain scans revealed that the amygdala and the anterior hippocampus showed greater activity when participants initially viewed negative images that they later remembered, compared to neutral images.

However, this heightened activity in these emotional memory regions for negative images diminished with repeated viewings. In contrast, for neutral images, activity in these areas remained relatively stable across repetitions. When examining the neocortex, the outer layer of the brain, researchers observed a distinct pattern. Brain regions in the front of the neocortex, including the inferior frontal gyrus and anterior temporal cortex, displayed an initial surge in activity for emotional memories, mirroring the amygdala and anterior hippocampus. This activity also decreased as the images were repeated.

Conversely, brain regions located in the back of the neocortex, such as the posterior temporal and parietal cortices, exhibited increasing activity over repetitions for neutral memories, but to a lesser extent for emotional memories. Beyond overall activity levels, the researchers investigated the stability of brain activity patterns. They discovered that for negative images that were successfully remembered, the patterns of brain activity in specific neocortical regions became more consistent across repetitions. These regions included parts of the prefrontal cortex, posterior parietal cortex, and posterior superior temporal sulcus. This indicates that the brain was essentially reinstating a similar pattern of activity each time a negative image was presented.

Further analysis indicated that this pattern stability in prefrontal cortex, superior parietal lobule, and posterior superior temporal sulcus was specifically linked to remembering individual emotional images, rather than just emotional images in general.

Finally, the researchers found that the initial activity of the amygdala when participants first saw a negative image played a key role in this process. A stronger initial amygdala response was associated with more stable brain activity patterns in the superior parietal lobule over repetitions, which in turn contributed to improved memory for those emotional images. This suggests that the amygdala’s initial emotional reaction triggers a mechanism that strengthens memory by enhancing the consistency of brain representations over time.

“The repeated exposure to emotional, compared to emotionally neutral, events leads to more robust and stable neocortical representations and this increase in pattern robustness is driven by the initial response of the brain’s major emotional processing center, the amygdala,” Schwabe told PsyPost.

The study, “Memory boost for recurring emotional events is driven by initial amygdala response promoting stable neocortical patterns across repetitions,” was authored by Valentina Krenz, Arjen Alink, Benno Roozendaal, Tobias Sommer, and Lars Schwabe.