A new study of the brain activity of people suffering from post-traumatic stress disorder (PTSD) is the first to show that traumatic memories are represented in the brain in a completely different way than sad autobiographical memories.
This finding supports the idea that traumatic memories in PTSD are a distinct cognitive entity distinct from regular memory, and it may provide a biological explanation for why the recall of traumatic memories frequently manifests as intrusions that differ significantly from “regular” negative memories in patients with post-traumatic stress disorder.
The study, conducted by researchers at the Icahn School of Medicine at Mount Sinai and Yale University, was also the first to examine people’s real-life personal memories rather than looking at basic cognitive mechanisms, in order to link personal experience to brain function.
“For people with PTSD, recalling traumatic memories often displays as intrusions that differ profoundly from processing of ‘regular’ negative memories, yet until now, the neurobiological reasons for this qualitative difference have been poorly understood,”
said senior author Daniela Schiller, Ph.D., Professor of Psychiatry, and Neuroscience, at Icahn Mount Sinai.
Trauma Recollection Brain Areas
The brain does not consider traumatic memories as regular memories, or perhaps as memories at all, according to the data collected by the researchers. It was observed that brain regions that are commonly associated with memory remain inactive during the recollection of a traumatic event.
“This finding provides a neural target and focuses the goals of returning traumatic memories into a brain state akin to regular memory processing,”
The formation and retrieval of episode memories are regulated by the hippocampus, according to prior research. Post-traumatic stress disorder (PTSD) is characterized by structural abnormalities of the hippocampus, most notably a reduction in volume. Moreover, the pathogenesis of PTSD is associated with impairments to hippocampal processes.
The posterior cingulate cortex (PCC) has been demonstrated to be heavily involved in both narrative comprehension and autobiographical processing and, particularly, in emotional memory imagery. Alterations in PCC function and connectivity are specifically focal to PTSD.
Autobiographical Memory Categories
To investigate whether and how the hippocampus and posterior cingulate cortex differentiate traumatic autobiographical memories from sad ones, 28 PTSD patients underwent autobiographical memory reactivation using script-driven imagery while undergoing functional magnetic resonance imaging (fMRI).
First, to generate stimuli based on participants’ individual autobiographical memories, the researchers used an imagery development procedure. The participants provided further details regarding three distinct categories of autobiographical memories:
- those classified as “PTSD” (traumatic memories of combat, sexual assault, domestic violence, etc.)
- “sad” (meaningful, yet non-traumatizing) (e.g., the loss of a family member or pet)
- “calm” (positive, tranquil) (representing memorable outdoor activities or bereavement)
These highly personal depictions of autobiographical memory were then systematically arranged into an audio clip approximately 120 seconds long, narrated by a member of the research staff.
Notably, the PTSD and sad storylines were written to be structurally identical to one other in order to control for content and arousal. While undergoing functional magnetic resonance imaging, participants listened to this innovative portrayal of their own memories for the first time.
Semantic Representation Differences
The team hypothesized that among people with PTSD, semantic similarity would lead to neural similarity: if two people’s personal memories are semantically similar, then their brain responses should be similar when they listen to audio recordings of these memories.
If traumatic and sad memories are simply distinct types of autobiographical recollections, the researchers expected to find semantic-to-neural connectivity between pairs of traumatic and sad memories. However, if traumatic autobiographical memories differ from sad autobiographical memories rather than being a version of them, then they would only observe the semantic-to-neural link for sad, but not traumatic, memories.
The research team was intrigued to find that patterns in the hippocampus showed differentiation in semantic representation by narrative type. In the hippocampus, sad scripts that were semantically similar across participants elicited similar neural representations on fMRI.
Conversely, thematically similar traumatic autobiographical memories did not elicit similar representations.
Importantly, the researchers discovered a positive relationship between semantic content and neural patterns of traumatic narratives in the posterior cingulate cortex, a brain region recently conceptualized as a cognitive bridge between world events and self-representation.
The research identifies a neural basis for the subjective experience of recalling a traumatic memory versus a routine recollection. According to the findings, a therapeutic goal aiming at “returning” the traumatic memory representation to a normal hippocampus representation may be advantageous.
For people with post-traumatic stress disorder (PTSD), recall of traumatic memories often displays as intrusions that differ profoundly from processing of ‘regular’ negative memories. These mnemonic features fueled theories speculating a unique cognitive state linked with traumatic memories. Yet, to date, little empirical evidence supports this view. Here we examined neural activity of patients with PTSD who were listening to narratives depicting their own memories. An intersubject representational similarity analysis of cross-subject semantic content and neural patterns revealed a differentiation in hippocampal representation by narrative type: semantically similar, sad autobiographical memories elicited similar neural representations across participants. By contrast, within the same individuals, semantically similar trauma memories were not represented similarly. Furthermore, we were able to decode memory type from hippocampal multivoxel patterns. Finally, individual symptom severity modulated semantic representation of the traumatic narratives in the posterior cingulate cortex. Taken together, these findings suggest that traumatic memories are an alternative cognitive entity that deviates from memory per se.
- Perl, O., Duek, O., Kulkarni, K.R. et al. Neural patterns differentiate traumatic from sad autobiographical memories in PTSD. Nat Neurosci 26, 2226–2236 (2023). Doi: 10.1038/s41593-023-01483-5