Researchers have now found out the fundamental mechanisms by which the brain consolidates remote fear memories or memories of traumatic events that occurred in the distant past — a few months to decades ago.

According to the study published in Nature Neuroscience, the remote fear memories formed in the distant past are permanently stored in connections between memory neurons in the prefrontal cortex, or PFC.

“It is the prefrontal memory circuits that are progressively strengthened after traumatic events and this strengthening plays a critical role in how fear memories mature to stabilized forms in the cerebral cortex for permanent storage,” said Jun-Hyeong Cho, an associate professor of molecular, cell and systems biology, who led the study.

The researchers concentrated on the PFC, a region of the cerebral cortex previously linked to remote memory consolidation.

“We found a small group of nerve cells or neurons within the PFC, termed memory neurons, were active during the initial traumatic event and were reactivated during the recall of remote fear memory,” Cho said.

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“When we selectively inhibited these memory neurons in the PFC, it prevented the mice recalling remote but not recent fear memory, suggesting the critical role of PFC memory neurons in the recall of remote fear memories,” he added.

Further, the researchers repeatedly exposed the mice to the same fear-predictive context but without the aversive stimulus to extinguish the mice’s remote fear memory. As a result, the fear response to the context was reduced.

“Interestingly, the extinction of remote fear memory weakened the prefrontal memory circuits that were previously strengthened to store the remote fear memory,” Cho said.

Moreover, Cho said that PTSD (post-traumatic stress disorder), which affects about 6 percent of the population at some point in their lives, can be caused by a dysregulation of fear memory consolidation.

“Considering that PTSD patients suffer from fear memories formed in the distant past, our study provides an important insight into developing therapeutic strategies to suppress chronic fear in PTSD patients,” he said.