Deficits in episodic memory and mental time travel in patients with post-traumatic stress disorder
Introduction
The episodic memory system allows us to encode specific autobiographical information of events that we have experienced in terms of “what happened,” “where it happened,” and “when it happened” (Dere et al., 2010, Pause et al., 2013, Zlomuzica et al., 2014). Episodic memories also contain perceptions, thoughts, emotions, and feelings we had during that experience. The concept of episodic memory has emerged as a central framework for examining the psychological and neurobiological processes that contribute to the development of PTSD. Current psychological treatment approaches of PTSD such as the (narrative) exposure therapy have revolved around the phenomena related to episodic memory in PTSD (Ehlers and Clark, 2000). Thus, a better understanding of aberrant episodic memory dysfunctions in the context of PTSD is important for both, the etiology and therapeutic management of this highly devastating disorder.
The idea that intrusions and flashbacks that are associated with PTSD might be a consequence of an undamped activation of a pathological episodic memory for the trauma experience has received considerable empirical support (Brewin, 2014, Isaac et al., 2006, Moradi et al., 2008). Similar to other emotionally relevant episodic memories, trauma-related memories are highly vivid and can be activated by either trauma-related stimuli or “spontaneously” due to retrieval-cue generalization leading to the patient's experience of re-living the traumatic episode (Brewin, 2015, Tulving, 2001, Tulving, 2002).
Although there is ample literature available that indicates episodic memory dysfunction in PTSD patients (Brewin, 2014, Dere et al., 2010, Isaac et al., 2006, Moradi et al., 2008) these findings are predominantly based on studies using tasks which do not necessarily capture the whole complexity of the episodic memory concept (Brewin et al., 2007, Isaac et al., 2006, Pause et al., 2010, Pause et al., 2013, Zlomuzica et al., 2014).
Apart from the remembrance of past experiences, the episodic memory concept involves the ability to perform mental time travel (MTT), to execute episodic future thinking and to establish prospective memories (Blix and Brennen, 2011, Breeden et al., 2016, Brown et al., 2014, Suddendorf, 2013, Suddendorf and Corballis, 1997). MTT is defined as the ability to recollect past events from episodic memory (MTT into the past) and to anticipate or imagine events in the future (MTT into the future). From a biological perspective, MTT seems to have evolved to serve a) the optimization of decision-making processes, b) efficient problem solving, c) the preparation for future needs, and d) the formation of intentions to perform actions at a specific time point in the future (Breeden et al., 2016). It is evident that patients with impaired MTT function would encounter problems in their social and professional functioning. Indeed, there is evidence that PTSD patients show difficulties in planning and structuring everyday activities (Mehnert et al., 2010, Scrignaro et al., 2011) and exhibit compromised social problem-solving abilities (Reich et al., 2015). However, it is unclear whether these difficulties can be attributed to impairments in MTT. Furthermore, very little work has been conducted to examine the integrity and functional significance of MTT, episodic future thinking and prospective memory in the context of PTSD (but see Kleim et al., 2014). Finally, the successful retrieval and use of specific personal experiences as a clue to solve anticipated future problems represents a central element of cognitive-behavioral treatment approaches for PTSD (Zlomuzica et al., 2014).
In order to examine these phenomena we developed a specific test for the utilization of episodic memories for solving problems, in the present and future, respectively. We used a VR environment to measure the ability to generate integrated memory for “what happened, where, and when” (Binder et al., 2015, Kinugawa et al., 2013, Pause et al., 2010, Pause et al., 2013, Zlomuzica et al., 2015). We tested PTSD patients and healthy controls in this VR-based episodic memory task as well as a newly developed clinical test for MTT, episodic future thinking, and prospective memory. The virtual reality episodic memory test (What-Where-When WWW Task) is based on the rationale of the episodic-like memory task we developed for rodents (e.g. Dere et al., 2005a, Dere et al., 2005b, Kart-Teke et al., 2006; Zlomuzica et al., 2007; reviewed in Binder et al., 2015), and was adapted to humans for computer-based testing (Kinugawa et al., 2013, Pause et al., 2010, Weber et al., 2014) and VR-based assessment (Zlomuzica et al., 2015). Interestingly, VR-based techniques have been increasingly used in the treatment of PTSD (Botella et al., 2015). Hence, investigating to which extent patients with PTSD and healthy subjects differ in their ability to perceive and process complex information in VR and disambiguate different contexts is of valuable interest.
In an attempt to overcome some of the previous methodological weaknesses when assessing mnestic functions in PTSD (see Isaac et al., 2006), age, education and sex-matched healthy participants were used as controls. A complementary neuropsychological examination of attention and everyday memory capacity (Moradi et al., 1999) was also conducted.
Section snippets
Participants
Twenty-one participants who fulfilled DSM-IV criteria for PTSD (4 males, 17 females) and 21 healthy controls (4 males, 17 females) without a psychiatric diagnosis participated in this study. The presence of an Axis-I diagnosis was determined for all participants by means of the Diagnostic Interview for Mental Disorders-short version (Mini-DIPS, Margraf, 1994). PTSD patients were recruited via board advertisements in two specialist treatment centers. Exclusion criteria for PTSD patients were: a)
Sample characteristics
As shown in Table 1, groups were matched in terms of age, gender distribution or level of education. As expected, PTSD patients experienced significantly more trauma events (MPTSD = 3.9 [SD = 2.1], Mcontrols = 0.7 [SD = 0.9], F(1,40) = 41.41; p < 0.001) and reported more repeated traumas than healthy controls (N = 16 in the PTSD group versus N = 2 in the control sample). The PTSD group also showed higher scores of intrusion, avoidance and arousal symptoms obtained with the IES (ps < 0.001) and PDS questionnaire (
Discussion
In the present study, we used a novel VR-based approach to investigate episodic memory formation and MTT in patients with PTSD. Compared to healthy controls, PTSD patients exhibited deficits in remembering item and temporal information both in the episodic memory formation and MTT tests. Consequently, PTSD patients showed impairments in their capability to employ information from episodic memory to solve current or future problems. This is in line with a recent report (using autobiographical
Conclusions
In sum, our findings suggest that PTSD patients are impaired in the capacity to establish and utilize episodic memories. Furthermore, the PTSD patients showed deficits in MTT, which might account for impaired social and professional functioning. We suggest that cognitive behavior therapy in PTSD should include interventions that train attention and working memory and refine the ability to utilize episodic memories to solve problems. Another important outcome of cognitive behavior therapy in
Acknowledgements
This work was supported by grant ZL 59/2-1 and ZL 59/2-2 of the Deutsche Forschungsgemeinschaft (DFG) to AZ and JM. MAdSS was supported by Heisenberg Fellowship SO 1032/5-1, and IN-SENS #607616. The funders had no role in study design, data collection and analysis, decision to publish, or preparation of the manuscript. The authors declare no conflict of interest.
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