記憶は思い出すたびに不安定になり、作り直される？ 怖い記憶を弱める研究のいま

Many people think a memory is like a saved computer file. But the brain may work differently. In 2000, scientists taught rats to fear a sound.

Later, the rats heard the sound again and remembered their fear. Right after that, the scientists gave them a drug in the brain. After that, the rats were not afraid of the sound anymore. When we remember something, the memory may become soft and be saved again.

It was once believed that a memory, after it has been fixed in the brain, stays stable. In 2000, however, researchers in New York trained rats to expect an electric shock after a sound. Later, they played the sound to bring back the memory and immediately put a drug that stops protein production into the amygdala, a fear center.

The rats no longer showed fear. When the drug was given without recalling the memory, or six hours afterward, the fear remained. This suggests that a recalled memory becomes unstable for a few hours and must be stored again. Researchers hope this could help people with painful memories, but results in humans have not been consistent.

In 2000, Nader, LeDoux and colleagues at New York University trained rats to link a tone with an electric shock. Later they played the tone alone to bring the memory back, and immediately infused a protein-synthesis inhibitor into the amygdala. The rats stopped showing fear.

Without the reminder, or when the drug came six hours later, nothing happened, and the result held even for memories fourteen days old. Recalling a memory, it seemed, made it fragile again until it was stored anew, a process called reconsolidation. The finding raised hopes for treating traumatic memories.

In 2009, people who took propranolol before recalling a learned fear showed no fear-boosted startle response the next day. In 2010, extinction training given ten minutes after reactivation, but not six hours later, kept fear from returning, in some cases for at least a year.

But many of these findings proved hard to replicate; a 2020 replication of the 2010 study found no benefit over ordinary extinction. A 2018 review co-written by Kindt listed possible reasons. Reminders that are too weak or too strong may not destabilize a memory. Laboratory memories differ from real trauma. And telling an erased memory from a hidden one is hard.