Computational Model of the Effects of Posttraumatic Stress on Hippocampal Volume

When Fear Shrinks the Brain: A Computational Model of
the Effects of Posttraumatic Stress on Hippocampal
Volume
Briana M. Smith,a,b Madison Thomasson,b Yuxue Cher Yang,b
Catherine Sibert,b Andrea Stoccob
aDepartment of Bioengineering, University of Washington
bDepartment of Psychology, University of Washington
Received 11 January 2020; received in revised form 24 April 2021; accepted 26 April 2021
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
Co-author Madison Thomasson is now at Department of Psychology, University of Nevada Reno
Correspondence should be sent to Briana M. Smith, Department of Bioengineering, University of Washington,
Campus Box 355061, Seattle, WA, 98915, USA. E-mail: [email protected]
500 B. M. Smith et al. / Topics in Cognitive Science 13 (2021)
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.
Keywords: Post-traumatic Stress Disorder; Hippocampus; Amygdala; Declarative Memory; Long-
Term Memory; ACT-R; Cognitive Architecture
1. Introduction
Post-traumatic stress disorder (PTSD) is a psychiatric disorder that is caused by
experiencing or witnessing a traumatic event, such as rape, domestic violence, assault, a
serious accident, or military combat. At the behavioral level, PTSD is characterized by persistent
avoidance and alterations in mood, as well as cognitive distortions surrounding the
trauma. One of the most characteristic and disruptive behavioral effects of PTSD, however,
is the unwanted re-experiencing of the trauma through nightmares, flashbacks, and/or
intrusive memories. Traumatic experiences evoke an emotional response that is accompanied
by increased activation of subcortical areas such as the amygdala (Liberzon & Sripada,
2008; Shin, Rauch, & Pitman, 2006). Intrusive memories are thought to occur because of the
simultaneous activation of the amygdala and hippocampus during memory encoding (Marks,
Franklin, & Zoellner, 2018). At the subcortical level, PTSD is also characterized by a marked
reduction in the volume of the hippocampus—a medial temporal lobe structure necessary
for memory functioning. It is important to note that this change is primarily structural, and
although often remarkably apparent, decreased hippocampal volume is not accompanied by a
functional impairment in long-term memory (LTM) performance (Karl et al., 2006).
The goal of this paper is to derive predictions about the changes in hippocampal volume
observed in PTSD by using a neurocomputational model to simulate intrusive memories
over time within an integrated cognitive architecture. The central idea of the model is that
intrusive memories operate within the context of a general theory of declarative memory,
specifically episodic memory. Within this framework, the maladaptive memory intrusions
observed in PTSD can be seen as the runaway process of an otherwise adaptive memory
system.