Traci Watson

Special for USA TODAY

In a breakthrough worthy of science fiction, researchers say they've devised a technique to change a lab mouse's bad memory of a particular place into a good memory, and vice versa.

Scientists have long known that the emotional content of a memory can sour from positive to negative — say, when a beloved vacation spot serves as the site of a breakup — or brighten from negative to positive. But in a study in Wednesday's journal Nature, researchers say that they've zeroed in for the first time on the brain circuits involved in such a transformation. And they manipulated precisely those brain circuits to switch a memory's emotional freight from pain to pleasure.

Memory "is actually not like a tape recorder or camcorder of the past. It's a reconstruction of the past every time you recall a memory," says neuroscientist Steve Ramirez, a co-author of the new study and a graduate student at MIT. The new findings are "a testament to how unreliable certain memories can be, despite our convictions."

At the start of the experiment, one group of mice, all of them male, got a mild electric shock, forming a negative memory. Then they were released into an experimental chamber roughly as big as a shirt box. They quickly learned that whenever they ventured into a certain section of the chamber — call it the target zone — they were flooded with the unpleasant memory of the electrical shock. Naturally enough, the mice avoided that part of the chamber.

The bad vibes were actually the handiwork of the scientists, who deliberately turned on the painful memory with the help of a laser that activated the brain cells where the painful memory was stored. The laser activated the electric-shock memory only in the target zone, ensuring the mice linked the negative emotion and the place. The laser light reached an animal's memory cells through a cable running from outside the body through the skull and into the brain.

The scientists then took the mice out of the chamber and gave each a sure-fire positive memory: quality time with two female mice. At the same time, the scientists used the laser to reactivate the bad memories to promote the swap from bad to good.

Then it was back into the chamber for the males. When the mice ventured into the target zone, the researchers again used a laser to fire up the brain cells where the negative memory had been stored. During this round, though, rather than fleeing the target zone, the mice lingered. The scientists say that's because the rodents' negative memories of the target zone had been replaced by pleasant memories, thanks to their recent dalliances with the ladies.

Similar experiments showed that happy memories could be displaced by unpleasant memories. But it's the replacement of bad memories by good that might be useful to people suffering from stress-related depression and other psychiatric diseases. It will be years before the new findings can be applied to humans, says study co-author Susumu Tonegawa of MIT, but he says he's "optimistic" about the prospects for new technology to translate the findings to people.

Other scientists praised the study as strong confirmation of long-held ideas about the locations in the brain where memories are stored. The new research is "a much cleaner demonstration than anyone had done before" that while much of the information about a place is stored in one brain region, the emotional freight attached to that place is stored in another region, says University of California-San Francisco, neuroscientist Loren Frank.

There are potential ethical issues with changing a patient's memories as a form of medical treatment, says Karen Rommelfanger, director of Emory University's Neuroethics Program. For example, would a memory change have side effects, perhaps on a patient's identity and interactions with others? But as long as the ethical issues are discussed early, Rommelfanger thinks it's worth pursuing these results, especially for their possible application to those suffering from post-traumatic stress disorder.

"There's a lot of potential for this to give us real insight into how the brain works and how we might help people," she says. "Something like this could be an important part of a therapeutic intervention."