Trauma-related intrusive memories (TR-IMs)—memories related to a traumatic event that come to mind spontaneously—are central symptoms of post-traumatic stress disorder (PTSD) and may be critical treatment targets for PTSD. Their frequency and intensity are usually measured with retrospective reports.
An alternative for assessing TR-IMs is ecological momentary assessments (EMAs), which are defined as repeated assessments of a particular behavior or phenomenon at strategically predefined times and over a given period. EMAs are considered more valid than retrospective reports for predicting behaviors in “real-world” settings.
Quentin Devignes, PhD, a research fellow in the Anxiety and Traumatic Stress Disorders Laboratory at McLean Hospital, and colleagues recently combined EMAs with neuroimaging to investigate TR-IMs in PTSD.
In Translational Psychiatry, they describe a critical role of the hippocampus in TR-IM neurophysiology.
Background
The anterior hippocampus (aHPC) and posterior hippocampus (pHPC) have partially distinct structural and functional patterns of connections with other brain areas. In line with this, they are thought to function differently in retrieving autobiographical memories.
The aHPC seems to play the principal role in the initial searching and accessing of memories, whereas the pHPC is more involved in elaborating episodic details.
The McLean study was the first to examine the relationship between the aHPC/pHPC structural covariance network and the severity of intrusion symptoms in PTSD. A structural covariance network represents the relationships between morphological features of different brain regions at the group level.
Methods
The researchers recruited trauma-exposed adults at McLean and the local community who were 18 to 65 years old, had been exposed to at least one DSM-5 criterion A trauma for PTSD, and reported at least two TR-IMs per week in the past month.
At their first visit, participants completed self-report questionnaires, including the PTSD Checklist for DSM-5 (PCL-5). Over the next 14 days, they were asked to complete five daily surveys via the MetricWire smartphone application (MetricWire Inc., Kitchener, Ontario, Canada). The current study only analyzed data from the three surveys about TR-IMs.
Participants who completed at least 70% of the surveys were invited to return for a second visit, which involved:
- The clinician-administered PTSD Scale for DSM-5 (CAPS-5)
- The clinician-administered Mini International Neuropsychiatric Interview (MINI)
- Self-report questionnaires
- MRI scanning for volumetric and structural covariance analyses
93 participants made up the final cohort. On average, they reported 21.35 TR-IMs over the two-week EMA period.
Results
Neither aHPC nor pHPC volume significantly predicted TR-IM frequency. However, the researchers identified a structural covariance pattern that was uniquely associated with aHPC volume (r, 0.60; P<0.001) and not with pHPC volume.
The expression of this pattern was negatively correlated with TR-IM frequency: each unit decrease in structural covariance was associated with an average 8.61% increase in TR-IM frequency (incidence rate ratio, 0.91; P= 0.002).
Said another way, a higher frequency of TR-IMs was associated with lower structural synchronization within the anterior hippocampus–cortical network involved in autobiographical memory.
Importantly, the association of structural covariance with TR-IM frequency remained significant after accounting for the severity of other PTSD symptoms by adjusting for the CAPS-5 composite score (IRR, 0.89; P<0.001).
Conclusions
Structural covariance network analysis, especially analysis of the anterior hippocampus, may be able to identify biomarkers associated with recurrent TR-IMs.
A priority for future research should be to explore whether this neural correlate represents a premorbid risk factor for greater TR-IM frequency, results from trauma exposure, or both.
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