People with post-traumatic stress disorder (PTSD) experience traumatic memories differently than other types of negative memories. A recent Nature Neuroscience study investigates the neural patterns of patients with PTSD while listening to narratives that depicted their own memories.
Study: Neural patterns differentiate traumatic from sad autobiographical memories in PTSD. Image Credit: Cat Box / Shutterstock.com
Background
Although personal memory plays an important role in PTSD, most neural-based research on PTSD has focused on the non-personal memories linked to learning and memory paradigms. To date, it remains unclear how traumatic memories differ from other adverse non-traumatic autobiographical memories.
It is imperative to understand whether traumatic memory is a robust manifestation of autobiographical memory or an entirely different representation. In this context, differences between individual traumatic narratives and characteristic experiences must be considered. From these instances, common markers linked to a trauma-driven state could also be identified.
Previous studies have indicated that events are bound across time and space to form episodic memories. The hippocampus plays an important role in tracking the sequences of events for the development of a narrative from discrete events, in addition to participating in the retrieval of episodic memories.
PTSD pathophysiology involves the impairment of hippocampal processes and is associated with structural abnormalities, particularly reduced volume, of the hippocampus. PTSD has also been associated with reduced functional connectivity between the hippocampus and default mode network (DMN) of other regions during rest.
Impaired hippocampal functioning could manifest the paradoxical mnemonic sequelae observed in PTSD. More studies are needed to understand how hippocampus-mediated mnemonic processes influence PTSD.
Retrieval of episodic memories commonly engages the amygdala and subsequently induces emotions. Similar to the hippocampus, the amygdala is also linked to PTSD manifestations, as it participates in hyperresponsivity to both threat-related and non-specific stimuli.
About the study
The current study determined whether the brain can differentiate between traumatic and autobiographical memories. To this end, the neural activities of patients with PTSD were evaluated while they were listening to their personal traumatic memories in the form of a fully annotated and structured audio narrative. Traumatic memories from each participant were compared to a negative but non-traumatic and sad memory, as well as a positive and calm memory.
It was hypothesized that similar semantics would induce the same neural activity across PTSD patients. Therefore, if sad and traumatic memories are different manifestations of autobiographical memories, semantic-to-neural correspondence across these memories would be expressed similarly. If traumatic autobiographical memories are different from sad memories, the semantic-to-neural relationship will only show a relationship with sad and not traumatic memories.
A total of twenty-eight participants were diagnosed with PTSD, which included eleven females. All participants were around 38 years of age and their clinician-administered PTSD scale (CAPS) score was around 41.2.
A highly personal and variable depiction of autobiographical memory was structured and arranged in an approximately 120-second audio clip, which was referred to as the narrative. Participants underwent reactivation of autobiographical memory using these scripted narratives while their neural patterns were simultaneously recorded through functional magnetic resonance imaging (fMRI).
Study findings
In the current study, patients with PTSD listened to a new rendition of their traumatic memory. Based on the duration and intensity of the stimuli, the paradigm was at the intersection of naturalistic narrative comprehension tasks and autobiographical memory reactivation.
The changes in neural representations were assessed during the processing of personal trauma narratives. These assessments were compared with negative non-traumatic narratives of the same individuals.
An inter-subject representational similarity analysis (IS-RSA) revealed that hippocampal patterns exhibit differentiation in semantic representation by narrative type. For example, semantically similar sad narratives exhibited similar neural representations across participants. However, thematically similar traumatic autobiographical memories did not induce similar neural activity.
In contrast to the hippocampus, the amygdala was not associated with semantic information in any notable manner. This indicates that the amygdala is linked with poor representational space for semantic content.
An exploratory analysis revealed an association between the posterior cingulate cortex (PCC) and individual PTSP symptom severity. This association highlights the extent of semantic-to-neural representation.
Importantly, the exploratory analysis did not identify any other regions of the DMN inducing similar functions.
Conclusions
The current study has some limitations, including a small sample size that limited the generalizability of the findings. This was more prominent in the ad-hoc analysis that was performed to analyze symptom severity, where the samples were divided into two halves. In addition, variance in the “calm” dataset was low, which could influence the semantic-to-neural relationship.
Despite these limitations, the current study extended the understanding of the real-life traumatic autobiographical memories in PTSD. The importance of the hippocampus and PCC in differentiating two types of autobiographical memory was also highlighted.
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