Introduction. Throughout life, people experience potentially traumatic events that can lead to psychopathological manifestations and psychological distress, including trauma-related anxiety disorders. During the first month after exposure to a traumatic event, an acute stress reaction (ASR) may occur, which may then transform into post-traumatic stress disorder (PTSD). PTSD is a heavy burden on the health care system of any country, negatively affects the physical and psychological well-being of a person, and, of course, impairs the social functioning of the victim.
Intrusive memories are one of the main symptoms of PTSD, they are anxious and obsessive, based on images of a traumatic event and are emotionally colored. PTSD models have shown that it is intrusive memories that can activate other PTSD symptoms. Based on this, it can be concluded that interventions aimed at overcoming intrusive memories will have a positive effect on other symptoms of PTSD.
Goal. Generalization of current data on the functioning of memory mechanisms underlying the occurrence of post-traumatic stress symptoms, systematization and comparison of theories of traumatic memory in recent years, and effective interventions that can be used to prevent PTSD.
Materials and methods. Qualitative and quantitative content analysis of English-language literature for the last 15 years was conducted on the main approaches to explaining the functioning of memory during traumatic events, the formation of traumatic memories. The search was performed using databases Pubmed, CrossRef.
Conclusions. This review summarizes the main theories that explain the occurrence of traumatic memories after a stressful event, the peculiarities of memory functioning and information processing. The ability to process traumatic memory is the basis for favorable post-traumatic adaptation. It is extremely important to develop behavioral interventions that can be mechanically controlled, low intensity and will be provided by non-specialists, will be easy to use, which will help to ensure their widespread use and accessibility.
Body of research
During the first month after exposure to a traumatic event, an acute stress reaction (ASR) can occur, which can then transform into post-traumatic stress disorder (PTSD) . PTSD is a heavy burden on the health care system of any country. It affects negatively the physical and psychological well-being of a person, and, of course, impairs the social functioning of the victim . Unfortunately, the onset of PTSD is often accompanied by comorbid mental and physical pathologies, such as depression , cardiovascular and endocrine diseases , drug use . Manifestations and severity of symptoms of post-traumatic stress disorder vary significantly depending on the geographical location and features of the culture of a particular region .
Unfortunately, there are no clear statistics on the prevalence of PTSD in Ukraine. Figures in the United States indicate that 20% of survivors will have PTSD. Each of 13 people in the United States will develop PTSD at some point in their lives. About 8 million adults in the United States suffer from PTSD during the year. 70% of adults in the United States experience at least one traumatic event in their lifetime .
According to European statistics, about two-thirds of the European population have experienced at least one traumatic event in their lifetime. 1 - 3% of Europeans in 2017 were diagnosed with PTSD (approximately 7.7 million people). Undoubtedly, the prevalence of PTSD varies considerably depending on the specific country of Europe .
The statistics on the prevalence of PTSD in women deserve special attention. Women are twice as likely to suffer from PTSD as men - 10% and 4%, respectively .
It should be added that the pandemic caused by coronavirus disease 2019 (COVID-19) had severe consequences for the psychological well-being of people around the world. For example, data from an online cross-survey among the Italian population showed (the number of respondents - 2332 people) that the percentage of PTSD symptoms was 29.5%. The high level of clinical stress symptoms allows us to consider a pandemic caused by COVID-19 as a traumatic event [2,12].
From January to February 2020, Chinese researchers conducted an online survey of the prevalence of post-traumatic stress symptoms in the most affected regions of China; the study included 285 participants. One month after the COVID-19 outbreak, the prevalence of post-traumatic symptoms was already 7%. Women reported significantly higher levels of PTSD symptoms in the areas of re-experiencing, negative changes in cognitive and emotional functioning, and overexcitation .
In general, any event is considered traumatic if it threatens to cause serious harm to the health or death of a person or others and causes feelings of severe fear, helplessness, or horror. The presence of a traumatic event is criterion A in the Diagnostic and Statistical Manual of Mental Disorders of the 5th revision (DSM-5)  for the diagnosis of PTSD. There are also stressful circumstances, which are so-called "non-traumatic life events", they are risk factors for the development of symptoms of PTSD . These events do not meet the DSM-5 criteria for traumatic events. Examples of such situations are problems related to employment, finances, difficulties in relationships, illnesses of loved ones, etc. A meta-analysis of 22 studies by Larsen and Pacella found that "non-traumatic" life stressors were indeed related to the severity of PTSD symptoms, but there was still a stronger association between traumatic events and the severity of PTSD symptoms. However, there is still a view that both traumatic and "non-traumatic" events can lead to the development of PTSD.
It should be noted that the negative impact of traumatic events on the physical well-being of the victim can be explained not only by the mediation of PTSD and other mental disorders but also regardless of mental state, as a consequence of physiological changes in the body in response to stimuli . This fact allows us to look at traumatic stress on a larger scale (as an independent predictor of physical health deteriorating), and not just through the prism of PTSD.
Potentially traumatic events are a powerful factor that causes psychological distress. The response to a potentially traumatic event is purely individual and depends on many factors. In particular, the socio-economic level , limited social support , the number of traumatic situations experienced in the past , effective coping strategies , and many other factors. In general, PTSD does not occur by accident - the criteria for the likelihood of injury are mediated by genetic and psychological determinants multiplied by the influence of society .
A large meta-analysis by Tang et al. , who studied the factor risk of PTSD in people who survived the earthquake, classified the factor into three main types: basic characteristics, trauma characteristics, and post-traumatic characteristics.
Starting with the main characteristics, a significant factor was gender, namely, gender differences in the response to traumatic events. Women are more sensitive to stress hormones, more sensitive to threats, less likely to use effective coping strategies, and more likely to interpret disasters more negatively than men . Biological and genetic factors are also decisive. For example, older children were more likely to develop PTSD after earthquakes than younger children. Low level of education and socio-economic status are risk factors for PTSD. People with higher education and socio-economic status can use better-coping strategies because they have greater social and economic resources. Trauma experienced in past history has been another risk factor for developing PTSD in adults, which can be partly explained by the idea that experiencing life-threatening traumatic events may increase the risk of mental disorders .
In terms of trauma characteristics, the following four risk factors were identified in this category: trapping experience, fear, injury, and loss. The experience of falling into a trap is extremely traumatic because of the feeling of helplessness, the feeling of complete lack of control over the situation. Circumstances such as debris left after the earthquake, destroyed buildings, and victims' bodies served as a constant reminder of the trauma and constantly restored post-traumatic symptoms . Obviously, fear was a significant predictor of PTSD, because this emotion is central to this disorder. In the case of an injury, especially if it was life-threatening , there is a direct connection - the more severe the injury (such as limb amputation), the more likely PTSD will occur . Also, if the injury has resulted in disability, it leads to a significant reduction in a person's quality of life and this loss of quality of life can lead to PTSD. However, it is noted in this metaanalysis that there were two articles that considered injuries on the contrary as a protective factor. This may be due in part to errors in these studies, and in part to the fact that the wounded people received more social support, which made them less susceptible to PTSD. Severe loss consumes a lot of resources, increases psychological stress, and can mediate the onset of PTSD.
Among the post-traumatic characteristics, loss of property, further financial difficulties were associated with an increased risk of PTSD. Also, risk factors were housing damage, job loss, insufficient social support, and declining socioeconomic status. For example, unemployment and the destruction of homes mean that a person cannot take care of their families as it was before the catastrophe. This fact leads to a significant level of distress.
A cohort study by Müller et al. aimed to identify risk factors associated with the occurrence of PTSD in US veterans . The sample consisted of 5,826 veterans who underwent regular clinical testing (using physical health indicators, demographics) between 1987 and 2011. The results showed that smoking, a history of chest pain, and younger age (age <59.4 years had the highest sensitivity and specificity for the onset of PTSD) were factors that contributed to the onset of PTSD.
In general, situations and events that can lead to psychological trauma can be classified as follows:
- acts of violence, such as armed robbery, war, or terrorism; natural disasters, such as fires, earthquakes, or floods;
- interpersonal violence, such as rape, child abuse, or suicide of a family member or friend;
- involvement in a serious accident or accident at work.
The diagnosis of PTSD first appeared in DSM of 3rd revision. In the 5th edition, it belongs to the section "Disorders related to trauma and stress".
The four main features of PTSD were unchanged, regardless of the issue number of the diagnostic and statistical manual :
- the presence of a traumatic stressful event;
- recurrence of symptoms of the event, including nightmares and (or) flashbacks;
- avoiding situations, places, and people that remind of a traumatic event;
- symptoms of excitatory tendency, such as irritability, difficulty concentrating, and sleep disturbances.
An innovation for the DSM-5 was the criterion of "negative changes in cognition and mood" - persistent negative beliefs and expectations about themselves, persistent distorted accusations of themselves or others, dissociative symptoms, feelings of alienation, and narrowing of affect .
Intrusive memories are one of the main symptoms of PTSD, they are anxious and obsessive, based on the fancy of a traumatic event, and are emotionally colored. Intrusive memories usually manifest in the form of visual images, although they may include sounds, smells, tastes, and sensations in the body . It should be noted that intrusive memories are also a symptom of acute stress reaction - from three days to one month after injury and PTSD - from 1 month after injury .
Moreover, PTSD models have shown that it is intrusive memories that can activate other symptoms of PTSD . Based on this, it can be concluded that interventions aimed at overcoming intrusive memories will have a positive effect on other symptoms of PTSD .
The question is, what is the relationship between experiencing a traumatic event and the symptoms of stress-related disorders? The answer is memory.
In order to understand the functioning of the memory mechanisms underlying PTSD, several models have been developed (Table 1, , , , , ). Although the discussion continues, many theories differ from standard views on memory function.
|Conditioning model||Mowrer/1947||Classical and operant conditioning: fear is caused by a traumatic experience, a combination of previous neutral stimuli with aversive and threatening stimuli.|
|Emotional processing theory||Foa & Kozak/1986||Fear is created by the fact that stimuli, responses, and semantic attributes are stored in the memory and form a non-adaptive network. A non-adaptive network perceives non-threatening stimuli as dangerous and generates inadequate responses to them.|
|Cognitive model||Ehlers & Clark / 2000||Injury treatment generates a sense of serious current threat due to two features: individual differences in the assessment of injury and/or its consequences; individual features in the nature of remembering a traumatic event and its relationship with other autobiographical memories of the person.|
|Theory of double representation||Brewin / 2014||There are two memory systems: verbally accessible memories, including memories of the context of the trauma that can be arbitrarily recalled and described; situationally available memory, which is limited to sensory memories caused by involuntary signals. During the traumatic experience, the lack of contextual processing compared to the received sensory information makes it difficult for the victim to tell about the trauma and integrate it into the autobiographical memory.|
|Memory-based model||Rubin / 2005||The development of PTSD is explained by the basic mechanisms of emotions, autobiographical memory, and personality traits of the victim. A number of independent systems (sensory, visual-spatial, linguistic, emotional, narrative, motor, explicit memory) interact to form autobiographical memories, and each part of this network plays a specific and important role in remembering events.|
In many models of PTSD, the resistance to the attenuation of conditioned fear is central. In the context of the conditionality of fear, attenuation involves the reduction of fear in response to a conditional danger signal that occurs one or more times in the absence of an unconditional stimulus. The lack of attenuation leads to the preservation of fear in response to stimuli that are no longer associated with danger, which indicates a maladaptive expression of anxiety . The attenuation of the conditioned reaction occurs only when the attenuation learning is strong enough to displace the memory of fear encoded during the acquired activation.
The two-factor Mowrer model, based on classical and operant conditioning, was one of the first models to try to explain the development of PTSD . The model is based on the idea that there are two learning processes: emotional conditioning, which arises from the combination of conditional stimuli with unconditional (according to Pavlov); and instrumental (or operant) conditioning, which arises by enhancing stimulus-response associations (according to Thorndike) by drive reducing (as in the drive reduction theory). Mowrer's theory emphasizes that none of the forms of learning are reduced to each other, but they can interact . Thus, in the case of PTSD, a traumatic event creates a scenario of fear, combining previously neutral stimuli with aversive and threatening stimuli. As a result, previously neutral stimuli may provoke a fear response in the future. The victim will believe that avoiding these conditional stimuli can prevent a fear response. This avoidance strategy is a negative reinforcement that supports the reaction of fear .
Emotional processing theory
To a large extent, the theory of emotional processing is based on the concept of "structures" of fear, developed by Lang . These structures are the specific mechanisms for the characteristics of stimuli (e.g., firearms), the characteristics of the response to stimuli (e.g., hyperventilation reaction) and the characteristics of semantic attributes (e.g., the thought "I'm going to die"), stored in the memory of the victim. The theory suggests that these structures work in PTSD in a specific way, identifying non-threatening stimuli as dangerous and thus generating an inadequate response to the stimulus. As in the conditioning model, avoidance plays an important role, but unlike the conditioning model, avoidance is not limited to behavioral mechanisms of avoidance . Cognitive avoidance (a strategy of not thinking about things that can evoke memories of trauma) and emotional numbness are added, which briefly reduces the activation of the fear system, keeping it in the long run .
This theory postulates that it is the memory mechanisms in which the above-mentioned “structures” of fear are represented play a key role in the development of PTSD . Peritraumatic dissociative states (e.g., derealization and depersonalization) lead to memory fragmentation and, as a consequence, to the malfunction of fear structures, which, in turn, will lead to an incorrect perception of stimuli in the future and further generalization of fear structures.
Exposure therapy used in the treatment of PTSD is aimed at breaking and reducing the link between the fear response and the stimuli associated with the traumatic event . The exhibition can be held in the imagination or situationally. In the first case, the victim is asked to imagine a traumatic situation during therapy, activating the fear response in such a way that the connection between the emotional response and the stimuli associated with the trauma does not diminish or disappear. During the situational exposure, the victim encounters traumatic stimuli in reality, but in a safe context. Thus, in both cases, there is a change from maladaptive memories of the trauma to more adaptive. Traumatic memory becomes autobiographical.
A key principle of the model developed by Ehlers & Clark is the presence of incorrect handling of a traumatic event in memory, which creates a sense of serious current threat. There are two main characteristics of the processing of information about a traumatic event:
- individual differences in the assessment of the traumatic event and/or its consequences;
- individual differences in the nature of memories of the event and their relationship with other autobiographical memories.
Regarding individual differences in the assessment of a traumatic event, the victim may have a mechanism of over-generalization - situations that were previously neutral can now be associated with fear reactions. A negative assessment of the consequences of a traumatic event creates a sense of current threat - the victim may perceive the symptoms of stress not as a natural part of recovery, but as an irreversible mechanism. Such features of injury assessment lead to maladaptive emotional and behavioral reactions.
Peculiarities of individual differences in the nature of memories of an event are based on the theory of double representation Brewin et al. . Memories are formed mainly due to sensory impressions, which are experienced as if they arise in the present tense (re-experience), as opposed to just thoughts about a past event. Also, the defining characteristics of these memories are:
- the memories are unprocessed and insufficiently incorporated into the autobiographical memory (the event is separated in the memory of the victim);
- have a strong stimulus-stimulus and stimulus-response relationship to the content associated with the trauma (memories of the trauma are easily evoked by a large number of stimuli);
- have a strong perceptual basis, i.e. there is a low threshold for processing memories that are associated with a traumatic event.
The differences in the assessment and nature of traumatic memory are related. When information about a traumatic event is mentioned, the true nature of the traumatic memory loses objectivity due to the previously given assessment of the traumatic event. That is, the information that is related to these assessments (which is a reflection of the subjective perception of the victim) appears in the memory.
Protocols for the treatment of PTSD with cognitive-behavioral therapy are based on the cognitive model. Several randomized controlled trials have demonstrated that individual cognitive therapy for PTSD is effective and acceptable for patients who have experienced a wide range of traumatic events [46-49]. Referring to the Ehlers & Clark model, the sense of threat is supported by three main processes. The first relates to the peculiarities of the formation of traumatic memory after an event. The second concerns the features of cognitive processing of the traumatic event by the victim, the individual way of assessing what happened, and the consequences. For example, if victims see themselves and their loved ones as more vulnerable to illness or death and do not trust health care providers, this will create a constant sense of threat. The third process that maintains a sense of current threat is maladaptive cognitive and behavioral coping strategies that the patient uses to reduce feelings of threat. These strategies may inadvertently aggravate symptoms (e.g., drug use to cope with symptoms) or a sense of threat (e.g., extreme vigilance against danger and excessive constant monitoring). It is worth noting that avoidance, protective behavior, and mental rumination prevent an overestimation of the importance that the victim has attached to the traumatic event or the nature of the memory of the injury . The cognitive model of PTSD proposed by Ehlers & Clark  is presented in Figure 1.
Theory of double representation
The theory of dual representation  focuses on explaining the functioning of memory and its role in the development of re-experience of the symptoms. According to this model, there are two memory systems that interact in parallel: verbally accessible memories (VAM) and situationally accessible memories (SAM). The VAM system includes oral and written information about traumatic situations, which forms an autobiographical memory that can be reproduced arbitrarily. VAM are presented in a coherent context, including information about the past, present, and future, including previous information that came to memory during and after the injury for storage in the long-term memory through conscious processing. Despite verbal accessibility, these memories are limited in the amount of information that can be consciously encoded.
Traumatic experience disrupts the conscious processing of information due to the fact that the victim pays attention to the immediate threat and a high level of emotional reactivity. This mechanism can explain the emergence of intrusive memories and flashbacks because these memories are mostly situationally accessible and involuntarily formed under the influence of external signals (e.g., sound transport) and internal signals (e.g., a certain emotional state) of a traumatic event. Situationally accessible memories contain information, the processing of which was accompanied by a low degree of awareness of the traumatic event. These memories are mostly focused on perceptual elements (sounds/images).
The SAM system is also responsible for recording physiological responses to injury (heart rate, sweating, temperature fluctuations, pain). Physiological manifestations make memories more intense, during the emergence of intrusive memories a person has the feeling that the traumatic event occurs "here and now" and is accompanied by the same sensations as at the time of the actual trauma. SAM are not encoded orally, so they are difficult to recount, process, and integrate into autobiographical memory. These memories are very difficult to control because people are likely to experience mnemonic symptoms that cause these memories involuntarily .
This model is used to justify and create preventive interventions in the early stages of memory consolidation. After all, the symptoms of PTSD can be reduced by processing VAM instead of SAM, which significantly reduces the symptoms from cluster B . In a study conducted by Holmes et al. , the traumatic film paradigm was used as an analog of PTSD. Participants first watched a traumatic film that contained scenes of real injuries and deaths, after which they had a 30-minute break. Next, they were randomly divided into two groups: one – which did not perform any tasks, the other – which performed a visual-spatial task for 10 minutes (playing Tetris). After that, study participants monitored the presence of involuntary memories for 1 week.
The final results showed that the band members who played Tetris after watching the movie had fewer flashbacks. According to the theory of double representation, this is due to the fact that the visual-spatial task (playing Tetris) competes with the touch processing of aversive video, which, in turn, is processed in context, through a system of VAM. Studies of the use of visual-spatial tasks for the preventive treatment of PTSD are extremely promising and relevant [52,53].
Rubin et al.  consider mnemonic phenomena associated with PTSD to be ideosyncrasic. The development of PTSD is explained by the basic mechanisms of emotions, autobiographical memory, and personality, which are called "the main mechanisms of vision". The memory model  emphasizes that when people experience a traumatic event, they then modify it. Memory is not a constant; it changes over time due to factors that affect memories and are related to individual differences (e.g., personality traits, gender, etc.). The interaction of these factors will determine the frequency of PTSD.
Rubin views memory as a complex system. Memory processing can be understood only using an approach that takes into account each involved basic system and their respective properties: cognitive, nervous, and behavioral .
Memory is processed using the following systems:
- sensory system - sight, hearing, smell, etc.;
- visual-spatial system related to the spatial arrangement of objects and people;
- motor system;
- explicit memory;
- search system that coordinates and links information with other systems .
The Rubin Basic Systems Model , which is a model of autobiographical memory, consists of a number of independent systems that interact with each other. Each system includes a network that contains its features (processes and forms of organization-specific to each system). The interaction of this network generates autobiographical memories, and each part of this network plays a certain and important role in remembering events.
Since there is a link between autobiographical memory and PTSD, the memory system as a whole is associated with a disorder, not just individual memories of a traumatic event. This view contradicts the notion of processing memories through a specific mechanism specific to PTSD. Although the diagnosis of PTSD requires the impact of a traumatic event, it is unlikely that trauma-related memories have a different set of mechanisms than those involved in the formation of general autobiographical memories .
Thus, Rubin  identifies three main factors that are decisive in the formation of memory after a traumatic event:
- the emotional tension of memory;
- when and how often this was mentioned in the past;
- how central the event is to the victim's life and identity.
The higher the frequency of recollection, voluntary or not, the greater the tendency to recall these memories in the future and the perception of memory as central. Therefore, centrality and repetition are associated with memory support, as well as greater intensity and emotional valence.
Autobiographical memory is dynamic, changing as we perceive new events (including potentially traumatic events). The interaction between the characteristics of the event and the processes of coding, storage, and recollection may contribute to more frequent and intense symptoms of PTSD.
Memory plasticity and its importance for PTSD prevention
Initial knowledge of memory plasticity was obtained through experiments using animal models . Memory consolidation is a process of memory stabilization that depends on time and is necessary to preserve it . Consolidation can be divided into two types - "synaptic" and "systemic". "Synaptic" consolidation is characterized by the strengthening of local neural connections due to a cascade of molecular processes involving protein synthesis and the formation of new synaptic connections. It occurs minutes to hours after encoding information. “Systemic” consolidation is, in fact, a reorganization of long-term memory across distributed brain chains that occurs within days or years of initial coding .
Das et al  investigated the molecular processes underlying memory consolidation, namely N-methyl-aspartate receptors (NMDA), which play an important role in synaptic consolidation. Nitric oxide blocks NMDA receptors, thereby potentially interfering with synaptic consolidation. The study found that individuals who inhaled nitrous oxide for 30 minutes immediately after watching the traumatic film, compared with the control group, showed a faster decrease in the frequency of intrusive memories within one week. Of course, the effect of nitrous oxide on the process of memory consolidation needs further research, but it can be concluded that the mechanisms of memory stabilization can be regulated by relatively simple procedures.
Re-stabilization of memory is a predictable process called “reconsolidation” of memory and requires the synthesis of de-novo protein . This process shows that there may be an additional window of opportunity that prevents memories of trauma from being preserved. It has been suggested that "new pharmacological and psychotherapeutic approaches aimed at memory consolidation should be a priority intervention to prevent the development of PTSD" [63,64].
The theory of memory consolidation assumes a time interval of several hours after the injury, during which the memory of the injury is plastic and vulnerable to violations. Studies of molecular and cellular mechanisms of memory consolidation in animal models have shown that stabilization of traumatic memory can be prevented shortly after exposure to the event . Cognitive science predicts that cognitive tasks with high visual-spatial requirements will selectively disrupt sensory (mostly visual) aspects of memory (i.e., those underlying intrusive memories) through competition for limited cognitive resources  when this memory labile. One recent study by Iyadurai et al. , investigated the impact of behavioral intervention after real traumatic events. The hypothesis was that the number of intrusive memories would be significantly reduced due to the use of a computer game with high visual-spatial requirements (Tetris), which causes a violation of the consolidation of memory sensory elements about the traumatic event. Three groups of subjects were formed: the main one, in which the intervention was performed on the basis of Tetris for 20 minutes (injury reminder signal + Tetris game); attention control group - placebo (subjects kept a written log of activity for the same period of time). In contrast to the study discussed above, in this study, the intervention was performed in the emergency department within 6 hours after the accident. The effect of the intervention on the number of intrusive memories was studied during the week following the injury (primary outcome) and 1 month after the traumatic event (secondary outcome). The results showed that during the first week after injury, the participants of the main group had fewer intrusive memories compared to the participants of the control group (initial result). After the first week in the follow-up period, the participants of the main group reported a decrease in the level of distress from intrusive memories in contrast to the comparison group. The study confirmed the achievements of previous studies conducted on laboratory models and confirmed the hypothesis of the effectiveness of tasks with high visual and spatial requirements in real traumatic situations .
Currently, there are many theories that explain the occurrence of traumatic memories after a stressful event, the peculiarities of memory function, and information processing. The ability to process traumatic memory is the basis for favorable post-traumatic adaptation [69,70].
It is extremely important to develop behavioral interventions , which can be mechanically controlled, low intensity and will be provided by non-specialists, i.e. will be easy to use, which will help ensure their widespread use and accessibility .
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