Why It's Hard to Shake Some Intrusive Memories

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New research published 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.

“Cognitive theories of post-traumatic stress disorder suggest that later intrusive memories of a traumatic event are partly caused by specific changes in how people felt, thought, and behaved during the event,” say the researchers, led by Alessandro Massazza of University College London. “We provide support for this hypothesis by showing that the specific moments of the traumatic event later experienced as intrusive memories are characterized by different ‘peritraumatic’ reactions to those moments from the same trauma that did not intrude later.”

A peritraumatic reaction, according to the researchers, describes the thoughts, emotions, and behaviors that people have during or immediately after a traumatic event. Some can be severe, such as dissociation (for example, thinking that what one is experiencing is not real or is a dream), distress (experiencing fear, horror, and/or helplessness during the event), and tonic immobility (not being able to move during the event).

To test whether intrusive memories about traumatic experiences were more likely to stem from certain types of peritraumatic reactions, the researchers recruited 104 survivors of the 2016-2017 Central Italy earthquakes to participate in a short in-person study. They asked the survivors whether they had experienced any intrusive memories about the trauma and, if so, to rate the memory on the following five dimensions of peritraumatic reactions: mental defeat, somatoform dissociation, cognitive overload, immobility, and distress.

“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,” says Massazza. “For example, an earthquake survivor might experience an intrusive memory of their collapsed house when passing by a construction site.”

The researchers also asked survivors to identify and describe other memories of the earthquakes that were just as distressing as intrusive memories but that had never spontaneously intruded. This way, they could compare the peritraumatic reactions of the intrusive and non-intrusive traumatic memories.

They found that severe peritraumatic reactions were more likely to appear in intrusive memories than non-intrusive memories.

“As hypothesized, moments experienced as intrusive memories were associated with higher levels of all peritraumatic reactions, except for mental defeat, when compared to moments from the same trauma experienced as distressing, but non-intrusive, by the same participants,” say the researchers. “These findings support the importance of peritraumatic encoding for intrusive memories.”

The researchers believe that these findings hold promise for developing new and effective forms of PTSD treatment.

“Our results have various practical implications for clinicians and people with lived experience of PTSD or trauma,” says Massazza. “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.”

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