05 Dec ​​Here is the discussion for memory models, how they have been applied to study PTSD and Alzheimer’s

​​Here is the discussion for memory models, how they have been applied to study PTSD and Alzheimer’s Disease, and how they could be used to study other disorders. 150 words.

 

 

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.

As memory is retrieved more frequently, its priority increases and its rate of decay

decreases. A traumatic memory, however, tends to out-compete more contextually appropriate

memories due to the fact it was