Where Do Intrusive Memories Come From?

Alessandro Massazza discusses his new research on intrusive memories and PTSD.

By Mark Travers, Ph.D. | September 28, 2021

A new article appearing in the Journal of Abnormal Psychology attempts to understand why certain memories from traumatic experiences are more likely to reappear as "intrusive memories" than others. The answer, according to the researchers, may have to do with a difference in the way intrusive memories are "encoded" into long-term memory.

I recently spoke with Alessandro Massazza, the lead author of the research, to discuss his findings in more detail. Here is a summary of our conversation.

What inspired you to investigate the topic of intrusive memories and PTSD and what did you find?

Intrusive memories represent a hallmark symptom of post-traumatic stress disorder (PTSD), a psychiatric disorder that some people can develop after very distressing events. Intrusive memories tend to be snapshots or brief snippets of the traumatic event that the person re-lives repeatedly and without wanting to. What interested us was why only certain snippets of a traumatic event are re-experienced as an intrusive memory whereas other moments from the same traumatic events are remembered like other normal memories from one's own life. We hypothesized, based on theories of PTSD such as the dual representation theory, that the way memory is formed (i.e., encoded) at the time of the traumatic event could play a role in determining which moments are later experienced as intrusive memories and which are not. The emotions, thoughts, and behaviors that people experience during trauma have been collectively named "peritraumatic reactions" and include phenomena such as dissociation (e.g., thinking that what one is experiencing is a dream) and tonic immobility (e.g., not being able to move). In line with predictions from theories of PTSD, we found that the moments later experienced as intrusive memories by earthquake survivors were characterized by higher levels of peritraumatic reactions in comparison to the moments from the same trauma experienced as non-intrusive memories.

How do you define an "intrusive memory"? What is the difference between an intrusive traumatic memory and a non-intrusive traumatic memory?

Intrusive memories are a symptom that is present across a variety of psychiatric disorders from major depressive disorder to obsessive compulsive disorder. In PTSD, they tend to be snapshots or brief snippets of the traumatic event that the person re-experiences repeatedly. Intrusive memories can only be remembered involuntarily and are triggered by internal (e.g., thoughts or emotions) or external (e.g., objects) things that bear some connection to the traumatic event. For example, an earthquake survivor might experience an intrusive memory of their collapsed house when passing by a construction site. This differentiates intrusive memories from normal memories from one's life which people are also able to recall voluntarily. Intrusive memories are also highly emotional memories with a lot of sensory detail. When experiencing intrusive memories, people often report being able to clearly see, smell, and hear what they experienced during the trauma. Importantly, in PTSD, intrusive memories are characterized by a degree of re-experiencing, which means that the person experiences the memory as if it were happening again in the here and now. This has led intrusive memories to be described as disjointed and disconnected from the rest of autobiographical memory (the memories from one's own life). This means that, even though the person has other memories from after the trauma indicating that the traumatic event has passed and that they are safe, they can still fully re-live the trauma as if they were still there.

Can you talk a little bit more about the idea of a "peritraumatic response" and how this fits into your conceptualization of PTSD?

The thoughts, emotions, and behaviors that people have during or immediately after a traumatic event have been collectively called "peritraumatic reactions." Our previous research has shown that there are many different peritraumatic reactions that people can experience during trauma. Some commonly studied reactions include dissociation (e.g., thinking that what one is experiencing is not real or a dream), distress (e.g., experiencing fear, horror, and helplessness during the event), and tonic immobility (e.g., not being able to move during the event). According to theories of PTSD such as the dual representation theory, these peritraumatic reactions can disrupt how memory is normally formed (i.e., memory encoding) and contribute to the development of intrusive memories. The idea behind those theories is that peritraumatic reactions can result in too much information being recorded on the emotions and sensations felt at the time of the trauma (sensory-affective information). In turn, this leads to less information being recorded on the context in which the trauma took place such as when and where the event happened (contextual information). This can result in memories where the sensory-affective information is not connected to its associated contextual information; where one can remember really well all the emotions and sensations experienced but not that the event has finished or happened in a different place. All this contributes to the development of highly emotional memories with a lot of sensory detail that the person re-lives in the here and now as they cannot place the memory in a specific time and place in the past.

What teachings might your research hold for people who have experienced trauma or have been diagnosed with PTSD?

Our results have various practical implications for clinicians and people with lived experience of PTSD or trauma. First, they suggest that peritraumatic reactions may play an important role in the development of intrusive memories, a hallmark symptom of PTSD. Therefore, interventions that aim at diminishing negative peritraumatic reactions during or immediately after a traumatic event (e.g., psychological first aid) are likely to hold potential in preventing the development of intrusive memories. Future work could also explore the possibility of training individuals at high risk of trauma exposure (e.g., fire-fighters or disaster first respondents) in the management of peritraumatic reactions. Additionally, our findings highlight that peritraumatic reactions such as dissociation can be commonly experienced to different degrees by trauma survivors. Clinicians working with trauma patients could play an important role in normalizing the experience of upsetting or strange peritraumatic reactions, such as not reacting during a traumatic event due to tonic immobility. Finally, an improved understanding of peritraumatic reactions may result in more personalized treatment choices given evidence highlighting how trauma survivors that experienced certain types of peritraumatic reactions may respond less well to certain evidence-based PTSD treatments.

Where do you hope to see this research go in the future?

Our research holds several limitations that future research could address. For example, participants recalled their peritraumatic reactions several months after the traumatic event had passed. Future research might attempt to collect data on peritraumatic reactions in real-time. For example, one study has measured dissociation and hyperarousal among novice skydivers immediately after the jump. More accurate ratings of peritraumatic reactions could also be achieved by studying these reactions in laboratory settings (e.g., by exposing individuals to analog traumatic events such as distressing movie clips) or by using technology to monitor peritraumatic reactions in real life (e.g., using Fitbits to monitor heart rate as a proxy measure for distress or dissociation among workers in high-risk professions). Our hope is that an improved understanding of peritraumatic reactions and of their relationship to intrusive memories will contribute to uncovering the mechanisms underlying a hallmark symptom of PTSD and to better prevention, treatment, and promotion of mental health among trauma survivors.