Mice react to fake memories: neuroscientists show how to synthesize recall of fear.
In the movie Eternal Sunshine of the Spotless Mind, scientists erase troubling memories from Jim Carrey's head. In real life, scientists have done the opposite.
By reactivating certain nerve cells, researchers can make artificial memories pop into mice's heads. The results, published in the March 23 Science and online March 22 in Nature, offer insights into how the brain creates and uses memories.
Memory research often studies natural memories or disrupts them. In the new work, memories are actually created, says neuroscientist Richard Morris of the University of Edinburgh. "This is an extremely important step forward."
Both teams created a false memory of a fearful situation in mice. The study in Nature, led by Susumu Tonegawa of MIT, used a genetic trick to mark memory-making nerve cells with molecules that respond to light. This allowed scientists to reactivate those cells later using light.
The team exposed the mice to shocks in one room. A day after the fearful experience, the animals were placed in an entirely different room. Yet, when the light was turned on and the artificial memory called to mind, the animals froze in fear. The flash of light "led to the entire recall of yesterday's terrible experience," says Tonegawa. Once the light was turned off, the mice moved normally.
In the work reported in Science, neuroscientist Mark Mayford and colleagues used a different method to mark the cells that formed a scary memory and reactivate those cells later.
Mice first explored a square room with opaque white walls and floor, and no particular odors. Researchers tagged these memories and then put the mice in a wintergreen-scented room with a black-and-white checkered wall. Here, the mice were subjected to shocks and learned to freeze.
Activating the memory of the odor-free room during the shock session taught the mice to associate the combination of the reactivated memory and the scented room with a shock, forming a hybrid memory. Later, these mice froze only when researchers placed them in the second room and simultaneously reactivated the artificial memory. "We've essentially created a synthetic memory," says Mayford, of the Scripps Research Institute in La Jolla, Calif.