Scientists have been capable of switching mice’s good memories with bad ones and vice versa. The discovery was the result of work done by a team, formed from a collaboration between Japan's RIKEN institute and the Massachusetts Institute of Technology in the US. The scientists injected two groups of male mice with light-sensitive algae protein, which enabled them to identify the formation of a new memory as it was happening and then use pulses of light to reactivate it when they wanted to. One group of rodents were allowed to play with female mice, creating a positive memory. The other group were given a small but unpleasant electric shock through the floor. Researchers then artificially reactivated the memory using the light pulses. While the mice were "remembering" their event, they were given the opposite experience -- the mice with the nice memory got a shock, while those with the painful memory were introduced to females. Research leader Susumu Tonegawa said his team had discovered that the emotion of the new experience overpowered the original emotion, rewriting how the mice felt about it. "These findings validate the success of current psychotherapy, by revealing its underlying mechanism," he said. "It depends on how strongly the (good or bad aspect) dominates... there is competition between the two circuits' connection strengths," Tonegawa said.