New Study Challenges Assumptions about How Memories Are Made

New research reveals that short-term memories and long-term memories are formed simultaneously in different parts of the brain, a discovery that challenges our current models of memory formation.

Short-term memories and long-term memories are formed simultaneously in different parts of the brain. Image credit: tasnimnews.com (CC BY 4.0)

Until recently, scientists believed that memories were first stored in the hippocampus then gradually “transferred” to the cortex for long-term storage.

However, a new study conducted by the Riken-MIT Center for Neural Circuit Genetics challenges this theory, suggesting that memories are simultaneously stored in the hippocampus and cortex, but remain “silent” in the cortex for about two weeks before reaching a mature state.

Scientists have assumed that the engram cells in the cortex that store memory were generated slowly. The cells actually develop rapidly, said Takashi Kitamura, a lead author of the study.

The past theory that short-term memories in the hippocampus transfer to long-term memories in the cortex was developed in the 1950s, when a famous amnesiac patient named Henry Molaison experienced a damaged hippocampus following surgery for epilepsy. He lost his ability to make new memories, but his long-term memories remained.

Kitamura and his team artificially reactivated memories in mice by using optogenetics, a technique that involves beaming light into their brains to control the activity of individual neurons by switching memories on or off.

Researchers used this approach to label memory cells during a fear-conditioning event that involved a mild electric shock to the mouse and then used light to artificially reactivate memories at different times.

Memory cells were labeled in three parts of the brain: the hippocampus, the prefrontal cortex and the basolateral amygdala, which stores memories’ emotional associations.

A day after the mice received an electric shock, there was evidence that memories of the fear-inducing event were stored in engram cells in both the hippocampus and the prefrontal cortex. The engram cells in the prefrontal cortex, however, remained silent.

These cells stimulated freezing behavior when artificially activated by light, but did not fire during natural memory recall. Furthermore, when the connection between the hippocampus and cortex was blocked, long-term memory was unable to mature.

As a result of the study, researchers discovered that memory engram cells naturally change their status from active to silent and from silent to active. If researchers can identify the mechanisms behind how overactive memory can become silent, they may find a way to inhibit overly strong, negative memories. (Tasnim News Agency)

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