If you've ever gone through a traumatic event, chances are very good that you remember it in vivid detail. According to research, however, the chances of you remembering less important details—like the song that was on the radio, the food you were eating, or the conversation you were having—are much, much lower. A new study used brain scans to find out why this happens, and the results could enable new treatments for conditions like post-traumatic stress disorder (PTSD).
For a study published in the journal Social Cognitive and Affective Neuroscience, researchers from the University College London put 20 volunteers in an fMRI brain scanner to analyze their brain activity while they looked at pairs of pictures they were asked to remember. Half of the pictures were of something negative, such as someone who had been badly injured, and the other half were neutral. That way, the randomized pairs ended up being a mix of two negative pictures, two neutral pictures, and one of each.
After seeing all the pairs, the participants took a 10 minute break, then looked at a series of individual images. Two thirds of the images were ones they had seen in the previous trial pairs. The trick was that for each image they saw, participants had to remember the image that had been paired with it in the first trial (they could also say they thought the image was entirely new).
Memory Without Context
The results? The volunteers were significantly better at recognizing negative pictures than neutral ones on their own. But in pairs, they were worse at remembering which pictures appeared alongside negative ones than neutral ones. The brain scans shed some light on this tendency: recognizing negative images was associated with increased activity in the amygdala, the part of the brain used for processing emotional information. Remembering which pictures appeared alongside the negative ones, however, was linked with reduced activity in the hippocampus, which forms associations among memories and helps retrieve every detail of a memory as a unit in the appropriate context.
In a press release, senior author Neil Burgess explained that this imbalance between memory of individual items and their surrounding details "could lead to strong but fragmented memory for the traumatic content of an event, without the surrounding information that would put it in the appropriate context. People who have suffered a traumatic event can experience vivid and distressing intrusive images from it, as in post-traumatic stress disorder. These intrusive images might occur due to strengthened memory for the negative aspects of the trauma that are not bound to the context it occurred in." That could explain why people with PTSD get flashbacks, and more importantly, may shed light on new treatments that could stop them from happening.