A new study co-led by Yale researchers finds that the brain activity triggered by recollections of traumatic experiences among people with PTSD is in fact markedly different from that which occurs when remembering sad or "neutral" life experiences.

In the study, which involved 28 different patients diagnosed with PTSD, researchers found that brain patterns were consistent across all individuals when they recalled their more typical life experiences.

But when reminded of traumatic events from their past, neural responses differed significantly among the individuals.

"When people recall sad or neutral events from their past experience, the brain exhibits highly synchronous activity among all PTSD patients," said Yale's Ilan Harpaz-Rotem, professor of psychiatry and psychology at Yale and co-senior author of the paper.

"They are not like memories at all."

The study, conducted with researchers at Icahn School of Medicine at Mount Sinai in New York, is published Nov. 30 in the journal Nature Neuroscience.

For the study, the researchers asked each of the 28 participants a range of questions, which pertained to their traumatic experiences, events in their lives that caused sadness (such as the death of a family member), and moments when they felt relaxed.

Each person's story was written down and then read back to them while they underwent fMRI (functional magnetic resonance imaging) scans, which are used to map brain activity based on blood flow.

The researchers found that activity in the hippocampus -- the area of the brain that forms memories of our experiences -- followed similar patterns of activity among all subjects when they were reminded of sad or relaxing experiences from their lives, suggesting typical normal memory formation.

But when stories about their traumatic experiences were read back to them, the similarities in hippocampal activity among the group members disappeared.

Instead, the hippocampus of each subject exhibited highly individualized and fragmented activity, unlike the more synchronous patterns of brain activity during normal memory formation.

The results could explain why PTSD patients have difficulty recalling traumatic experiences in a coherent way and hints at why these past experiences can trigger disabling symptoms, the researchers say.

These insights may help psychotherapists guide PTSD patients to develop narratives about their experiences which may help them eliminate the sense of immediate threat caused by their trauma, Harpaz-Rotem said.