Those who have been traumatized by an event often cannot control their thoughts about the experience and relive the traumatic situation over and over again. A study now points to a possible neural cause for this: According to this, traumatic memories are processed differently in the brain than normal sad memories. The brain activity is more fragmented, varies greatly from person to person and does not show the typical patterns for retrieving memory content. From the researchers’ perspective, the results support therapeutic approaches that aim to transform traumatic memories into normal memories.
People who have experienced traumatic events such as sexual assault, domestic violence, serious accidents, or war often develop symptoms of post-traumatic stress disorder (PTSD). They cannot control their thoughts about the event, experience anxiety and nightmares and often suffer from so-called intrusions, in which they suddenly feel like they have been transported back to the traumatic situation.
Focus on neurobiological processes
A team led by Ofer Perl from the Icahn School of Medicine at Mount Sinai in New York has now examined how memories of traumatic events differ from memories of other sad events. “In the experience of people with PTSD, intrusions feel fundamentally different than normal negative memories,” explains Perl’s colleague Daniela Schiller. “Until now, however, the neurobiological reasons for this qualitative difference were poorly understood.”
Previous studies have already shown that in people with PTSD, the hippocampus has reduced volume and its function is impaired. This brain region is active, among other things, when we recall episodic memories. In addition, it was already known that the posterior cingulate cortex, which is involved in narrative comprehension and autobiographical processing, is also altered in PTSD. However, studies that focus on the individual experiences of those affected have so far been lacking.
Individual memories
For the current study, Perl and his team asked 28 people with PTSD to tell the research team three different autobiographical episodes: the traumatic experience, a meaningful sad event that was not traumatic, such as the death of a family member, and a calm, neutral one to positive situation, for example a walk in the forest or a relaxing vacation. Based on the personal reports, the researchers put together an individual audio recording for each test subject, in which a member of the research team recounted the respective situations. The traumatic and sad experiences were formulated as similarly as possible in order to increase comparability.
These recordings were played to the subjects while their brain activity was recorded using functional magnetic resonance imaging (fMRI). The team particularly observed activity in areas associated with memory, including the hippocampus and posterior cingulate cortex. “When memories of sad or neutral events were evoked, the brains of all test subjects showed highly synchronous activity,” reports co-author Ilan Harpaz-Rotem from Yale University in New Haven. This is consistent with the assumption that narratives that are worded in a similar way also evoke similar reactions in the brain.
Fragmented brain activity
The situation was completely different with traumatic memories: “When the test subjects were presented with stories about their own traumatic experiences, the brain activity was highly individualized, fragmented and disorganized,” says Harpaz Rotem. From the researchers’ perspective, this could explain why those affected find it difficult to structure their thoughts about a traumatic event.
“Our data show that the brain does not treat traumatic memories as normal memories, or perhaps even as memories at all,” says Schiller. “Our results support the idea of a profoundly separate cognitive experience in the reactivation of traumatic memories. This is consistent with the idea that traumatic memories are not experienced as memories per se, but rather as fragments of previous events that displace the present moment.”
Implications for psychotherapy
One goal of psychotherapy for people with PTSD is for those affected to learn to integrate the traumatic event into their life story and to gain control over their memories. The current study provides a neurological explanation as to why such therapies can be helpful: “A neural goal could be to return the traumatic memories to a brain state that is similar to normal memory processing,” says Schiller.
Source: Ofer Perl (Icahn School of Medicine at Mount Sinai, New York) et al., Nature Neuroscience, doi: 10.1038/s41593-023-01483-5