New research has found that, in people with PTSD, the brain processes traumatic personal memories differently from sad ones, activating a region of the brain normally associated with things like introspection and daydreaming. The findings may be critical to PTSD treatment.
Personal – or autobiographical – memories are at the core of PTSD. However, it’s not clear how traumatic autobiographical memories differ from negative but non-traumatic ones. Previous research has demonstrated that the brain’s hippocampus has a role in the construction and retrieval of episodic memories, memories of everyday events, including associated emotions, that can be explicitly stated or recreated.
A new study by researchers at the Yale and Mount Sinai schools of medicine examined whether and how the brain differentiates between traumatic and sad autobiographical memories.
The researchers recruited 28 participants diagnosed with PTSD, who underwent reactivation of an autobiographical memory by listening to a scripted audio recording of it while undergoing functional magnetic resonance imaging (fMRI).
The participants were asked to elaborate on three types of autobiographic memories: the ‘PTSD’ condition, a traumatic event associated with their PTSD, such as sexual assault or domestic violence; the ‘Sad’ condition, a meaningful but non-traumatic experience like the death of a family member or pet; and the ‘Calm’ condition, a positive, calm event. These memories were then arranged into an approximately 120-second audio clip narrated by one of the researchers, scripted to maximize their structural similarity.
The researchers’ analysis of the memory narratives suggested that sad and traumatic memories overlapped in terms of themes and semantic content, laying the foundation for asking whether the neural patterns associated with these two memory types would differ. Semantic memory refers to general world knowledge people have accumulated over their lifetime. While semantic memory is distinct from episodic memory, both are types of explicit or declarative memory that can be recreated at any given time.
All participants listened to each narrative three times during the fMRI scan, which measured brain activity in the hippocampus, amygdala, and posterior cingulate cortex (PCC). Emotional memories often engage the amygdala, and its activation during memory encoding modulates the memory’s subsequent strength. The PCC encodes and retrieves episodic and autobiographical memories.
The scans revealed clear differences between memories. Listening to sad memories showed consistently high engagement of the hippocampus, whereas, in traumatic memories, the hippocampus was not involved. The traumatic memories appeared to engage the PCC, with more severe PTSD symptoms producing more activity.
“Our main finding, that hippocampal patterns of PTSD patients showed a differentiation in semantic representation by narrative type during memory reactivation, supports the idea of a profoundly separate cognitive experience in the reactivation of traumatic memories,” said Ilan Harpaz-Rotem, one of the study’s corresponding authors. “This is consistent with the notion that traumatic memories are not experienced as memories per se. Rather, these are fragments of prior events, subjugating the present moment to evade the comfort of belonging to the past.”
In other words, when it comes to traumatic memories, the brain appears not to be in a state of memory; rather, it’s in a state of present experience.
The researchers provide several possible explanations for this. One is that people with PTSD may develop a highly detailed and very personal memory of their traumatic event, making their semantic representations highly unique to them despite the memory being semantically similar to other memories. Another possibility is that traumatic memory reactivation is not experienced as memory as such but is disconnected from time, space and current surroundings and experienced as “an intrinsic mental event.” Lastly, patients may have attempted to block or suppress the reactivation of traumatic content, exhibiting brain activity that was not in keeping with other non-traumatic memories.
In addition to providing evidence that traumatic memories have distinct pathways in the brain, the findings highlight the involvement of the PCC, an area normally associated with internally directed thoughts, such as introspection or daydreaming. They also suggest that reactivating a traumatic memory may be critical to PSTD treatment as it helps transform it into an ‘ordinary’ sad memory that can be organized and consolidated into the hippocampus.
The study was published in the journal Nature Neuroscience.
Source: Yale School of Medicine
