Scientists have successfully hacked into the brains of napping mice, implanting false but cheerful memories via electrode stimulation, and demonstrating for the first time that memory manipulation during sleep is possible.

The team from the National Centre for Scientific Research (CRNS) in France managed to create positive feelings about a specific location that five mice had explored earlier in the day. They did this by stimulating their brains as they slept. So strong were these feelings, that when the mice woke up, they immediately wandered back to that location, presumably seeking some kind of reward.

“The mouse develops a goal-directed behaviour to go towards the place,” lead author Karim Benchenane told NewScientist. “It proves that it's not an automatic behaviour. What we create is an association between a particular place and a reward that can be consciously accessed by the mouse.”

The results of the study have been published in Nature Neuroscience.

The idea of memory manipulation is totally reminiscent of Michel Gondry's film Eternal Sunshine of the Spotless Mind, where the protagonist and his former love interest both have the painful memories of their relationship wiped clean. Predictably, when certain things (objects, songs, letters, etc.) trigger old, no-longer-existent memories, a bit of confusion ensues.

It’s an interesting, if somewhat troubling premise, but in future, with more testing and less invasive procedures, it’s exactly where memory manipulation technologies like this one could find applications.

“The idea is to use this as a tool for post-traumatic stress disorder,” Benchenane told Hannah Devlin from The Guardian.

“I think this is a really important step towards helping people with memory impairments or depression,” Loren Frank, a neuroscientist from the University of California, San Francisco who wasn't involved in the study, told Jessica Hamzelou from NewScientist. “In principle, you could selectively change brain processing during sleep to soften memories or change their emotional content.”

The researchers wanted to test the hypothesis that mice, like humans, consolidate memories as they sleep, replaying the day's events and learning in the process.

In order to insert their false memories, the researchers needed to first locate specific neurons, or place cells, within the mice’s brains that fire signals and help form spatial memories about specific locations.

The researchers implanted electrodes into the hippocampus - the region of the brain associated with memory formation - of five mice. The electrodes recorded neural activity while the mice explored a new environment, and the researchers were able to identify spikes in activity associated with specific locations.

Each mouse was then monitored during a one-hour nap. When place cells began firing it suggested to researchers that the mouse was recalling its experience from earlier in the day. Each time these cells fired, the researchers used a separate electrode to stimulate a region of the brain associated with pleasure.

As Hamzelou from NewScientist reports: “When the mice awoke, they made a beeline for the location represented by the place cell that had been linked to a rewarding feeling in their sleep. A brand new memory - linking a place with reward - had been formed."

Neuroscientist Neil Burgess at University College London, who wasn’t involved in the research, told NewScientist it was a "bigger breakthrough" than previous studies, adding: “The mouse is remembering enough abstract information to think ‘I want to go to a certain place', and go there when it wakes up.”

The researchers say one of the next steps is to see if the same results can be achieved during rapid eye movement sleep - a stage of sleep the mice didn't enter during their short one-hour siesta.

Other teams are also bringing us closer to the realisation of memory manipulation. In August 2014, scientists in the US used lasers to flip negative memories into happy ones in mice, and DARPA has an ambitious program to restore memories in veterans who have suffered traumatic brain injuries.

Sources: NewScientist, The Guardian