Remembering the past is an important function and defines who we are. In some situations though, the normal processes that store our experiences into memory can go wrong. After experiencing a distressing event, people can develop memory disturbances where they re-experience the event in the form of flashbacks – distressing vivid images that involuntarily enter consciousness, as happens in post-traumatic stress disorder.

Our latest study shows that a distressing experience has opposite effects in two different parts of the brain: the amygdala and the hippocampus. The amygdala, a region of the brain involved in emotion, seemed to strongly encode the negative content of an experience while the hippocampus, which is involved in storing new memories, is only weakly activated.

When remembering something from the past, we can bring to mind what we were doing, the people we were with, and where the event took place. An important aspect of memory is that these separate pieces of information are bound together as a single memory so that all of it can easily be recalled at a later time. But when experiencing a distressing event, the normal processes that help to integrate this information in memory can be disrupted.

The hippocampus is crucial for forming these associations so that all parts of a memory can be later retrieved as a single event (and damage to this brain region can stop a person from forming new memories). In contrast, the amygdala is involved in processing emotional information and making basic responses to things associated with fear, such as recoiling from a snake or spider.

People who have suffered a trauma often have difficulty remembering the context of the event. We thought that, while processing in the amygdala might be increased during a negative experience, processing in the hippocampus might be decreased, disrupting the way it binds the different aspects of the experience together as a single memory.

Brain imaging study

To test this idea we showed 20 volunteers pairs of pictures and asked them to remember the pictures while lying in an MRI scanner. Some of the pictures were of traumatic scenes, such as a badly injured person.

The volunteers’ memory of the pictures was then tested in two ways. First, they were shown one picture from each pair and asked if they recognised previously seeing it. Second, if the picture was recognised, we then asked whether they could remember what other picture had been part of the original pair.

When asked whether they recognised the individual pictures, people showed better memory for previously seen pictures that were negative (traumatic) compared with pictures that were neutral, such as a person sitting at an office desk. Improved memory for negative pictures related to increased activity in the amygdala. In contrast, their memory for remembering what pictures were presented together as a pair was worse when one of the pictures was negative.

We also found that activity in the hippocampus was reduced by the presence of negative pictures suggesting that its function in storing the associations between the pictures was impaired. This imbalance could lead to strong memories for the negative content of an event that is not properly stored with the other parts of the event and the context in which it took place.

Implications for psychotherapy

This work supports the view that experiencing a traumatic event might alter how memory works. The re-experiencing of intrusive images in post-traumatic stress disorder might happen because of strengthened memory for the negative aspects of a trauma but not their context – that is, the location where the event occurred or the time it occurred. This may result in the person involuntarily retrieving the traumatic event “out of context” and experiencing it as though it was in the present.

In this case, therapy should focus on strengthening or recreating appropriate contextual associations for the negative event. This view is supported by current psychotherapies where a person is taken back to the place where the traumatic event took place to help in strengthening memory for the context.

These findings also highlight potential issues with eyewitness testimony as trauma sufferers with poorly contextualised memories are likely to provide a fragmented report of an event.