Sleeping minds: prepare to be hacked. For the first time, conscious memories have been implanted into the minds of mice while they sleep. The same technique could one day be used to alter memories in people who have undergone traumatic events.

When we sleep, our brain replays the day’s activities. The pattern of brain activity exhibited by mice when they explore a new area during the day, for example, will reappear, speeded up, while the animal sleeps. This is thought to be the brain practising an activity – an essential part of learning. People who miss out on sleep do not learn as well as those who get a good night’s rest, and when the replay process is disrupted in mice, so too is their ability to remember what they learned the previous day.

Karim Benchenane and his colleagues at the Industrial Physics and Chemistry Higher Educational Institution in Paris, France, hijacked this process to create new memories in sleeping mice. The team targeted the rodents’ place cells – neurons that fire in response to being in or thinking about a specific place. These cells are thought to help us form internal maps, and their discoverers won a Nobel prize last year.

Benchenane’s team used electrodes to monitor the activity of mice’s place cells as the animals explored an enclosed arena, and in each mouse they identified a cell that fired only in a certain arena location. Later, when the mice were sleeping, the researchers monitored the animals’ brain activity as they replayed the day’s experiences. A computer recognised when the specific place cell fired; each time it did, a separate electrode would stimulate brain areas associated with reward.

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.

This must be the place

It is the first time a conscious memory has been created in animals during sleep. In recent years, researchers have been able to form subconscious associations in sleeping minds – smokers keen to quit can learn to associate cigarettes with the smells of rotten eggs and fish in their sleep, for example.

Previous work suggested that if this kind of subconscious learning had occurred in Benchenane’s mice, they would have explored the arena in a random manner, perhaps stopping at the reward-associated location. But these mice headed straight for the location, suggesting a conscious memory. “The mouse develops a goal-directed behaviour to go towards the place,” says Benchenane. “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 mouse is remembering enough abstract information to think ‘I want to go to a certain place’, and go there when it wakes up,” says neuroscientist Neil Burgess at University College London. “It’s a bigger breakthrough [than previous studies] because it really does show what the man in the street would call a memory – the ability to bring to mind abstract knowledge which can guide behaviour in a directed way.”

Benchenane doesn’t think the technique can be used to implant many other types of memories, such as skills – at least for the time being. Spatial memories are easier to modify because they are among the best understood.

His team’s findings also provide some of the strongest evidence for the way in which place cells work. It is almost impossible to test whether place cells function as an internal map while animals are awake, says Benchenane, because these animals also use external cues, such as landmarks, to navigate. By specifically targeting place cells while the mouse is asleep, the team were able to directly test theories that specific cells represent specific places.

“Even when those place cells fire in sleep, they still convey spatial information,” says Benchenane. “That provides evidence that when you’ve got activation of place cells during the consolidation of memories in sleep, you’ve got consolidation of the spatial information.”

Benchenane hopes that his technique could be developed to help alter people’s memories, perhaps of traumatic events (see “Now it’s our turn”, below).

Loren Frank at the University of California, San Francisco, agrees. “I think this is a really important step towards helping people with memory impairments or depression,” he says. “It is surprising to me how many neurological and psychiatric illnesses have something to do with memory, including schizophrenia and obsessive compulsive disorder.”

“In principle, you could selectively change brain processing during sleep to soften memories or change their emotional content,” he adds.

Nature Neuroscience