In 2013, scientists in the United States and Japan made a false memory in mice. First, a mouse stayed in a safe room, Room A. The scientists marked the brain cells that remembered this room.

Then the mouse went to Room B. There, the scientists turned on those cells with light and gave a small shock. Later, the mouse was afraid in Room A, but nothing bad ever happened there.

A technique called optogenetics allows scientists to switch specific brain cells on with light. In 2012, a team at MIT and RIKEN, led by the Nobel Prize winner Tonegawa, marked the cells in the hippocampus that were active when mice formed a fear memory in one room.

Later, when light was shone on these cells in a different, safe room, the mice froze in fear. In 2013, the team went further. They marked the cells that stored the memory of a safe room, then activated them with light while giving the mice a mild shock in another room. Back in the safe room, the mice froze, even though nothing bad had ever happened there.

A mouse enters a room where nothing bad has ever happened and freezes in fear. In 2013, researchers at MIT and RIKEN, led by the Nobel laureate Tonegawa, did exactly this by creating a false memory. Their tool was optogenetics, in which a gene for a light-sensitive protein from algae makes neurons fire only when lit. In 2012, the team had tagged neurons in the hippocampus that were active while mice learned to fear a room where they received shocks.

Later, in a different, harmless room, shining light on those cells made the mice freeze. Switching on one specific group of cells, called an engram, was apparently enough to trigger recall. This time they tagged the engram for a safe room, A, and reactivated it with light while the mice received mild shocks in room B. Back in room A, the mice froze, though they had never been shocked there.

The method has obvious limits. It needs genetically modified animals with implanted optical fibers, and the memories were simple. Artificial reactivation may also differ from how human false memories arise. Even so, recalling the false memory engaged brain regions much like those used for genuine fear memories.