When Fear Shrinks the Brain: A Computational Model of the Effects of Posttraumatic Stress on Hippocampal Volume

Abstract

Post‐traumatic stress disorder (PTSD) is a psychiatric disorder often characterized by the unwanted re‐experiencing of a traumatic event through nightmares, flashbacks, and/or intrusive memories. This paper presents a neurocomputational model using the ACT‐R cognitive architecture that simulates intrusive memory retrieval following a potentially traumatic event (PTE) and predicts hippocampal volume changes observed in PTSD. Memory intrusions were captured in the ACT‐R rational analysis framework by weighting the posterior probability of re‐encoding traumatic events into memory with an emotional intensity term I to capture the degree to which an event was perceived as dangerous or traumatic. It is hypothesized that (1) increasing the intensity I of a PTE will increase the odds of memory intrusions, and (2) increased frequency of intrusions will result in a concurrent decrease in hippocampal size. A series of simulations were run and it was found that I had a significant effect on the probability of experiencing traumatic memory intrusions following a PTE. The model also found that I was a significant predictor of hippocampal volume reduction, where the mean and range of simulated volume loss match results of existing meta‐analyses. The authors believe that this is the first model to both describe traumatic memory retrieval and provide a mechanistic account of changes in hippocampal volume, capturing one plausible link between PTSD and hippocampal volume.

Memory, Interrupted: A Retrieval Model of Intrusive Memories, Recovery Trajectories, and Neurobiological Effects in Post-Traumatic Stress Disorder

“…This lumps models runs that either never developed intrusive memories (Resilient) or successfully recovered after developing them (Recovery) into the "Control" group, and runs that either experienced intrusive memories throughout the run (Chronic) or developed them later on (Delayed) in the "PTSD" group. Note that this analysis departs from our previously reported work (Smith et al, 2021) which instead focused on the main effect of one external (intensity of the traumatic event, I PTE ) and one internal (cognitive control, W) factor. A similar analysis was conducted for the predicted functional connectivity between the medial prefrontal cortex and the hippocampus, operationalized as the degree of similarity between retrieved memories and current context and visualized in Figure 16.…”

“…Specifically, we will focus on PTSD-related changes in the size of the hippocampus and in its functional connectivity to the prefrontal cortex. These two measures capture consistent and key abnormalities reported in the literature, and they can be approximated in the model as the value of Shannon's information entropy, H, across all memories (Smith et al, 2021) and the mean value of the similarity between the retrieved memory and the contextual cues.…”

“…This product is analogous to the concept of expected utility in decision-making (Schoemaker, 1982), which is defined as the product of an outcome's probability by its utility. In this case, a memory represents an outcome, its probability is the memory's posterior probability of being re-encoded, and its "utility" is its COMPUTATIONAL MODEL OF INTRUSIVE MEMORIES importance (Smith et al, 2021). Following on ideas of rational analysis, a memory's "importance" is supposed to reflect its utility for the survival of an organism; in this sense,…”

“…As a proxy for hippocampus size, we calculated the entropy. This method was described in detail in Smith et al (2021). In their formulation, the size of the hippocampus is related to the number of memories and their relative activation level.…”

“…However, as noted by Garfinkel and Liberzon (2009), cortisol levels are not universally or consistently elevated in individuals with PTSD; indeed, potentially COMPUTATIONAL MODEL OF INTRUSIVE MEMORIES even suppressed (Morris et al, 2016;Pan et al, 2018). The model's estimates, instead, rely on a different logic, namely, that the hippocampus size reflects the landscape of memory availability and that, in PTSD, the dominant availability of the traumatic memory implicitly causes a reduced number of hippocampal cells and synapses devoted to storing memory information (Smith et al, 2021).…”