The first step was to create a memory. They did this by stimulating a group of nerves in the rat's brain (which equated to the sound of a tone) that had been genetically modified to respond to light, while shocking the animal's foot at the same time. From the rat's perspective, the sound of the tone (done by stimulating the nerve cells) was equated with the fear of getting a mild shock.
Then the team weakened the connection between the brain cells, which had the effect of erasing that memory. But in a twist, they were also able to recover the fear-pain memory by strengthening the synaptic connection by stimulating the synapse with a different frequency.
"We can form a memory, and then turn it off and turn it on by selectively turning on synapses," said Robert Malinow, professor of neurosciences and an author on the paper. "It puts together a number of things we have known and learned to produce this effect. It reinforces that synapses are important and can control memory."
Malinow says the finding could open the door to manipulating the creation of memories in humans as well. In PTSD, memories of certain traumatic events cause severe anxiety, depression and other problems in patients, while Alzheimer's disease causes us to lose our memories.