According to the International Statistical Classification of Diseases and Related Health Problems, version 10 (ICD-10), and the Diagnostic and Statistical Manual of Mental Disorders 4th edition-revised (DSM IV-R), intrusive memory experiences (IMEs) are a core criterion for post-traumatic stress disorder (PTSD) diagnosis. In that context, characteristic PTSD symptoms are influenced by confrontation with trauma-associated triggers arising from all senses. Triggers must be differentiated in adverse IMEs as scenes or sensual experiences that lead to IMEs, which are defined as re-experiencing symptoms of parts of a trauma within the PTSD context. These experiences appear to be temporary, with strong inter-individual variations in the perceived intensity and extent of the re-experienced trauma (Kleim et al., 2013; Berntsen and Rubin, 2014; Muller et al., 2015).

Post-traumatic stress disorder affects up to 3% of the general population, and by definition, persists for at least 1 year (Ohayon and Shapiro, 2000; Davidson et al., 2002; Green, 2003).

The DSM defines PTSD as a hallucinogen-like, persistent perception disorder without explicitly citing the cause of the trauma (Bourne et al., 2013; Bryant et al., 2013; Nickerson et al., 2013). Recurrent memories of a traumatic experience vary among the visual, acoustic, tactile, olfactory, and gustatory perception levels and strongly differ in intensity, duration, and frequency. Improved ability to predict IME quality is thus desirable. Therefore, substantial variability exists in the traumatic event as the underlying cause of PTSD development and the trauma experience, including subjective evaluation. PTSD patients can be categorized into three groups: single-trauma patients, re-traumatized patients (single or multiple repetitions of an earlier trauma) and patients who have been traumatized over a period of years and suffered from a number of different traumatic experiences (Taubenfeld et al., 2009; Holmes et al., 2010; Bryant et al., 2011; Besser et al., 2014). Existing research provides limited knowledge regarding this heterogeneous group’s re-living of traumatic experiences through IMEs at the specific perceptual level (Hellawell and Brewin, 2004; Duke et al., 2008; Muller et al., 2015). Little is known about factors determining the quality of such experiences at the predominant perceptual level. Research on this topic has covered several areas, including examination of the underlying trauma; the command language used to represent different types of trauma; and biological aspects, such as changes in regional cerebral blood flow (Osuch et al., 2001), functional magnetic resonance imaging (fMRI) (Lanius et al., 2005) changes and changes in gender– and psychosocial-specific aspects. Several studies have shown the profound, prolonged impact of Holocaust-related traumatization (Cohen et al., 2001; Golier et al., 2006; van Rooij et al., 2014; Carmel et al., 2016). Re-traumatization is believed to cause greater PTSD symptoms than single traumatization does (Schock et al., 2010). However, few data are available for direct comparisons of these different types of trauma (Muller et al., 2015). Thus, our research objective was to compare these special traumatization groups.

The role of triggers in IMEs is also important to explore. In addition to spontaneous occurrence of IMEs, sensory stimuli known as triggers are frequently responsible for eliciting IMEs in single, re- and prolonged traumatization (Ehlers et al., 2004; Kleim et al., 2013). However, the sensory quality and threshold intensity of triggers that elicit IMEs may differ (Biermann et al., 2010; Muller et al., 2011; Post et al., 2014). Nevertheless, considering the factors influencing symptom variety might be crucial to individual therapy and to our understanding of PTSD (Holmes et al., 2010; Muller et al., 2015). We hypothesize that the frequency and quality of IMEs differ among patients with single, re- and prolonged traumatization. Although data from this study population have been published before (Muller et al., 2015), the present study reports extensive results on specific questions regarding these IME.

Therefore, the current study sought to explore the differences in trauma perceptions according to sensory modalities of IMEs among the three different groups (patients with single trauma/re-traumatization/prolonged traumatization).

The results revealed a highly significant difference between the levels of trauma experience that induced PTSD and the frequency of IMEs (p < 0.01).

In the prolonged traumatization group of Holocaust survivors, the traumatic experiences manifested for these individuals either through committing physical and emotional violence against themselves or through witnessing violence against another individual. In both the single- and re-traumatization groups, the majority (75%) of trauma experiences were a combination of personal experiences of physical and emotional violence in the form of potentially life-threatening accidents and personal physical (single-trauma group: n = 9; re-traumatized group: n = 11) or emotional (single-trauma group: n = 6; re-traumatized group: n = 4) damage. In five cases in each group, the patient was an immediate witness of serious violence to another person. The frequency distribution of IMEs in the three groups of patients with single trauma, re-trauma, and prolonged trauma is shown in Table 1. In the prolonged traumatization group of Holocaust survivors, the majority (59%) experienced IMEs several times per day. Having IMEs several times per week or per month was more prevalent among patients with single traumas or re-traumatization (p < 0.05).

The distribution of primary perceptual qualities for IMEs and triggers in the three patient groups is shown in Table 2.

A primary symptom of PTSD is the patient’s re-experiencing of the initial trauma through nightmares, hallucinations, emotional crises, and IMEs (Taubenfeld et al., 2009; Belleville et al., 2012; Bryant et al., 2013; Cloitre et al., 2013). This study demonstrated that the perceptive quality of IMEs is significantly associated with other clinical symptoms, such as the frequency of IMEs. The severity of traumatization was significantly associated with IMEs at several perceptual levels. Additionally, the types of stimuli that could trigger IMEs were highly dependent on the trauma experience. The most frequent triggers among Holocaust survivors with prolonged traumatization were visual and combined visual/acoustic stimuli. Purely visual trigger stimuli were most common among single-trauma patients, and combined visual/acoustic triggers were most common among re-traumatized patients. Despite efforts to establish a diagnostically uniform entity, PTSD must be considered heterogeneous in its pathogenesis and manifestation. The consequences of the afflicted person’s exposure to trauma differ widely according to whether the person experienced single, recurrent or prolonged exposure over a period of years (Green, 2003; Hellawell and Brewin, 2004; Duke et al., 2008).

The trauma images or IMEs at various perceptual levels extend beyond the one-dimensional vision of an ever-recurrent pictorial memory (Hellawell and Brewin, 2004; Bryant et al., 2013; Cloitre et al., 2013). IMEs that result from sensory trigger sensations do not appear to uniformly exhibit equivalent perceptual levels. Visual triggers can elicit non-visual IMEs, and acoustic stimuli can elicit visual IMEs. In the present study, this was especially evident in the re-traumatization group, in which 50% of patients had visual IMEs only, while triggers were both visual and acoustical for 90% of patients. This diversity of perceptual dominance within the context of PTSD symptoms contributes to explaining the necessary complexity of therapeutic and scientific considerations (Brady et al., 2000; Tucker et al., 2001; Keeshin and Strawn, 2014). The current study sample differs from that of previous studies that have primarily considered other specific groups of PTSD patients, such as World War II veterans, victims of rape or sexual assaults, or victims of other trauma (Green, 2003; Jones et al., 2003; Duke et al., 2008). In the group of Holocaust survivors with prolonged traumatization, IMEs most frequently occurred several times per day, whereas the single-trauma and re-traumatized patients generally experienced IMEs several times per week or month. Jones et al. (2003) noted that the perceptive quality of IMEs might be affected by the media and other technological advances, leading to increased plasticity of IME perception in various senses. Our study did not identify age- or gender-related differences in IME perception or frequency. This outcome was surprising because one could hypothesize that the influence of media may be stronger among younger PTSD patients. However, the absence of age and gender differences might be related to the small size of our study. Prolonged traumatization, with wide varieties of permanent personal or shared existential threats, results in more intensely pronounced quantitative re-experiencing of trauma than does single- or re-traumatization. This result might be expected because the incidence of PTSD is strongly related to the frequency and severity of traumatization (Howgego et al., 2005; Durai et al., 2011; Hunt et al., 2011). However, we did not assess trauma severity, which is a limitation of our study. In all three groups, visual perception predominated in the perceptual quality of IMEs. However, a combined form of acoustic and visual memories in IMEs was considerably more frequent among re-traumatized and prolonged traumatization patients than among single-trauma patients. With increasing duration, the perception of the event is represented in perceptual qualities that are more varied than in cases of single trauma, with visual perceptions being predominant. In patients with single traumatization, there was a high correlation between the perceptual quality of IMEs (in this case, visual) and the perceptual quality of the eliciting trigger as visual stimuli. In contrast, greater variance in the perceptual quality of IME triggers with combined acoustic and visual stimuli was found in patients with prolonged traumatization. In re-traumatized persons, the nearly exclusive stimulation of IMEs elicited by the combined visual and acoustic perception quality of the trigger stimuli was surprising. Another limitation of our examination was the mean age and gender heterogeneity among the three different groups; however, we controlled for this characteristic of the groups using LSD post hoc testing, which is suitable for random samples of that size. Holocaust survivors are elderly. Thus, the rater might have assumed that an older patient has been traumatized by the Holocaust and thus categorized them as having prolonged traumatization. Additionally, age might have affected memory functions, especially among those with prolonged traumatization. However, patients with comorbid mental illnesses, such as dementia, and patients with other medical conditions that could affect memory functions were excluded from the study. Nevertheless, comorbid mental disorders were excluded using chart diagnosis, which could lead to inclusion of patients with additional mental disorders other than PTSD. Another potential source of bias is the use of retrospective questionnaires to assess IMEs; this method may not be the most appropriate assessment method and may be subject to recall bias of unknown magnitude. We used only one rater, which could potentially lead to a systematic bias. Additionally, the role of multiple traumatic events versus long-term traumatic exposure was not completely examined using this study design. Future controlled studies should address these concerns.

The multiplicity of perception engramming and the sensory modality of perception activation increase along with the frequency and duration of trauma, as previously described (Durai et al., 2011; Hunt et al., 2011). This current examination can encourage future studies to assess how the intensity and modality of IMEs influence overall treatment and outcomes. Patients with prolonged traumatization more frequently suffer from visual and combined visual/acoustic IMEs, whereas visual IMEs occur most frequently in single-trauma patients.

Regarding events triggering IMEs, trauma experiences and the occurrence of IMEs, the elicitation of IMEs by perception-specific triggers differs among those with single, re- and prolonged traumatization. Whereas the primary visual perceptual quality of the single trauma was maintained in the trigger, the eliciting perception spectrum of triggers among patients with re- and prolonged traumatization expanded to combine visual and acoustic stimuli.

In addition to the result regarding IMEs combining both visual and acoustic types associated with increased trauma frequency and duration, the described phenomenon indicates an expansion of trigger-specific elicitation of IMEs through the combined visual and acoustic perceptual contents. Hellawell and Brewin (2004) addressed this phenomenon in relation to ordinary autobiographical memories. They found that the perceptual descriptions of IMEs were more detailed, with mentions of death, use of the present tense and fear, helplessness and horror; in contrast, ordinary memories involved secondary emotions, such as guilt or anger (Hellawell and Brewin, 2004). The trauma-dependent quality of the perception of IMEs and triggers, as described in the current population, indicates that the multiplicity of perception engramming and sensory modalities of the perception activation (trigger) increase with the frequency and duration of trauma (Durai et al., 2011; Hunt et al., 2011).

Post-traumatic stress disorder patients who were Holocaust survivors more frequently suffered from visual and combined visual/acoustic IMEs, whereas visual IMEs were most common among single-trauma patients. Regarding IME triggering events, the trauma experience and the IME trigger stimulus are related. From a neurobiological perspective, the question of whether re- and long-term prolonged traumas play different roles and have different cerebral neurogenesis correlates than single traumas remains unanswered. Future research should address these questions and consider how they may contribute to improving the understanding and treatment of different types of PTSD.

HM and WS designed the study. KJ, AS, and KW provided intellectual input and participated in the data analyses. HM and SM wrote the manuscript. WS, SM, and HM critically revised the manuscript. All authors have read and approved the final manuscript.

The authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.