What are remote fear memories? How are they stored and retrieved?

A mouse study demonstrated that remote fear memories formed in the distant past are permanently stored in connections between memory neurons in the prefrontal cortex, or the PFC.

But what are remote fear memories? A remote fear memory is a memory of traumatic events that occurred in the distant past - a few months to decades ago.

"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," explained the lead researcher Jun-Hyeong Cho.

How are remote fear memories stored and retrieved? According to the UC Riverside study, the brain uses distinct mechanisms to store recent versus remote fear memories. Previous studies have suggested that while the initial formation of fear memory involves the hippocampus, it progressively matures with time and becomes less dependent on the hippocampus.

The researchers focused on the PFC, a part of the cerebral cortex that has been implicated in remote memory consolidation in previous studies.

"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," said Cho.

Could we prevent the recall of these remote fear memories? "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," explained Cho.

"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, other manipulations that blocked the strengthening of the PFC memory circuits also prevented the recall of remote fear memory," said Cho.