Modeling Memory Dynamics in Post Traumatic Stress Disorder: Intrusive Memories, Recovery Trajectories, Neurobiological Effects, and Fear Overgeneralization

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 thesis presents a neurocomputational model using the Adaptive control of Thought - Rational (ACT-R) cognitive architecture that simulates intrusive memory retrieval following a potentially traumatic event (PTE) and derives predictions about an individual’s recovery trajectory, neurological effects including changes in hippocampal volume and functional connectivity, and behavioral symptoms. Memory intrusions were captured in the ACT-R framework by weighting the prior probability of re-encoding a memory by an emotional intensity term I, which captures the degree to which an event was perceived as dangerous or traumatic. A series of simulations were run in which the model performed memory retrieval under naturalistic conditions for up to two months after experiencing a simulated PTE. It was found that I had a significant effect on the probability of experiencing traumatic memory intrusions following a PTE, and that, under different conditions, the model experienced different probabilities of undergoing different recovery trajectories. 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. Functional connectivity between the medial prefrontal cortex and hippocampus was shown to be significantly greater in the simulated control group than in the simulated PTSD group. Finally, the model accounts for the behavioral effect of fear overgeneralization in PTSD, predicting greater startle probability to novel compound stimuli after a fear conditioning task in simulated PTSD groups compared to a simulated control group. The authors believe that this is the first model to describe traumatic memory retrieval, predict recovery trajectories, and provide a mechanistic account of multiple symptoms of PTSD including intrusive memories, neurobiological changes in hippocampal volume and functional connectivity, and fear overgeneralization.