At the root of post-traumatic stress disorder, or PTSD, is a memory that cannot be controlled. It may intrude on everyday activity, thrusting a person into the middle of a horrifying event, or surface as night terrors or flashbacks.

Decades of treatment of military veterans and sexual assault survivors have left little doubt that traumatic memories function differently from other memories. A group of researchers at Yale University and the Icahn School of Medicine at Mount Sinai set out to find empirical evidence of those differences.

The team conducted brain scans of 28 people with PTSD while they listened to recorded narrations of their own memories. Some of the recorded memories were neutral, some were simply “sad,” and some were traumatic.

The brain scans found clear differences, the researchers reported in a paper published on Thursday in the journal Nature Neuroscience. The people listening to the sad memories, which often involved the death of a family member, showed consistently high engagement of the hippocampus, part of the brain that organizes and contextualizes memories.

When the same people listened to their traumatic memories — of sexual assaults, fires, school shootings and terrorist attacks — the hippocampus was not involved.

“What it tells us is that the brain is in a different state in the two memories,” said Daniela Schiller, a neuroscientist at the Icahn School of Medicine at Mount Sinai and one of the authors of the study. She noted that therapies for PTSD often sought to help people organize their memory so they can view it as distant from the present.

“Now we find something that potentially can explain it in the brain,” she said. “The brain doesn’t look like it’s in a state of memory; it looks like it is a state of present experience.”

Indeed, the authors conclude in the paper, “traumatic memories are not experienced as memories as such,” but as “fragments of prior events, subjugating the present moment.”

The traumatic memories appeared to engage a different area of the brain — the posterior cingulate cortex, or P.C.C., which is usually involved in internally directed thought, like introspection or daydreaming. The more severe the person’s PTSD symptoms were, the more activity appeared in the P.C.C.

What is striking about this finding is that the P.C.C. is not known as a memory region, but one that is engaged with “processing of internal experience,” Dr. Schiller said.

The findings feed into a much debated question in the field of trauma: Should clinicians encourage people with PTSD to expose themselves to their most traumatic memories?

In recent years, many Americans have embraced treatments such as prolonged exposure therapy and eye movement reprocessing and desensitization, or EMDR, which revisit traumatic memories in hopes of draining them of their destructive force. Ilan Harpaz-Rotem, an author of the paper, said the new findings suggested that revisiting the memory was a critical element of treatment.

“You are helping the patient to construct a memory that can be organized and consolidated into the hippocampus,” said Dr. Harpaz-Rotem, a professor of psychiatry and psychology at Yale University.

He described a case from his clinic: An Army medic was haunted by a fragmentary image from his past, of frantically bandaging a solder’s wound while under fire. In therapy, trying to “build a story, a coherent memory,” the clinician helped the medic fill in details around the edges of that scene, including a dead soldier who lay nearby, shooting in the background, and his own panicked use of too many bandages.

Ideally, such treatments can help transform the traumatic memory into one that more closely resembles ordinary sad memories. “It’s like having a block in the right place,” he said. “If I can access a memory, I know it’s a memory. I know it’s not happening to me now.”

Dr. Ruth Lanius, the director of PTSD research at the University of Western Ontario who was not involved in the study, described its findings as “seminal,” both because it establishes that traumatic memories have distinct pathways and because it indicates that key mechanisms for traumatic memory may involve less-examined areas of the brain. Much research into PTSD has focused on the amygdala, the stress detection center of the brain, and the hippocampus, she said. The posterior cingulate cortex is “really involved in the reliving of memories,” and in seeking self-relevance, which may explain why a sensory reminder may cause overwhelming fear or panic.

“A soldier, if they hear fireworks, they may run and take cover,” Dr. Lanius said. “Traumatic memories are not remembered, they are relived and re-experienced.”

Clinicians, she said, can use these findings to treat patients who “don’t feel that the trauma is over,” employing therapies that “bring on line context, so you know, ‘Oh, that happened in the past.’” She said researchers should explore therapies, like mindfulness, which are known to activate the parts of the brain known to provide context.

If biological markers for PTSD can eventually be identified, it would be “a major scientific contribution,” settling differences within the field about what experiences constitute a trauma, said Brian Marx, deputy director of the Behavioral Science Division of the National Center for PTSD, who was not involved in the study.

While most experts agree that motor vehicle accidents, sexual assaults or military combat are traumatic events, there is disagreement about whether experiences like racism or pandemic stress should be viewed as the basis for a PTSD diagnosis, he said.

“It is one of the foundational questions of the field,” he said. “It is a debate we still wrestle with, because we don’t have an answer for it.”

Dr. Marx called the new research “intriguing,” but not conclusive, noting that it did not include a comparison group of subjects without a PTSD diagnosis, specify how long ago the traumatic events took place, or specify whether the subjects had already received psychotherapy.

And he said it was not likely to settle debates over whether PTSD treatments should include exposure to traumatic memories, since literature on treatment outcomes show that responses are highly individualized.

“To say this is proof positive really ignores the reality that our treatments are imperfect,” he said. “They don’t work for everyone in the same way.”

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