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鈥檚 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 鈥� an essential part of learning. People who miss out on sleep do not learn as well as those who get a good night鈥檚 rest, and when the replay process is disrupted in mice, so too is their ability to remember what they learned the previous day.
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 (Nature Neuroscience, ). The team targeted the rodents鈥� place cells 鈥� neurons that fire in response to being in or thinking about a specific place. These cells are thought to help us form internal maps; last year their discoverers won a Nobel prize.
Advertisement
鈥淚t鈥檚 a bit worrying. It implies you could make someone want something even if they didn鈥檛 really鈥�
Benchenane鈥檚 team used electrodes to monitor the activity of mice鈥檚 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鈥� brain activity as they replayed the day鈥檚 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 鈥� linking a place with reward 鈥� had been formed.
鈥淚t鈥檚 unbelievably hard to do any of this鈥� but it鈥檚 not impossible that it could happen鈥�
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 鈥� for example, smokers can learn to .
Previous work suggested that if this kind of subconscious learning had occurred in Benchenane鈥檚 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. 鈥淭he mouse develops a goal-directed behaviour to go towards the place,鈥� says Benchenane. 鈥淚t proves that it鈥檚 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.鈥�
鈥淭he mouse is remembering enough abstract information to think 鈥業 want to go to a certain place鈥�, and go there when it wakes up,鈥� says neuroscientist at University College London. 鈥淚t鈥檚 a bigger breakthrough [than previous studies] because it really does show what the man in the street would call a memory 鈥� the ability to bring to mind abstract knowledge which can guide behaviour in a directed way.鈥�
This must be the place
Benchenane doesn鈥檛 think the technique can be used to implant many other types of memories, such as skills 鈥� at least for the time being. Spatial memories are easier to modify because they are among the best understood.
His team鈥檚 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.
鈥淓ven when those place cells fire in sleep, they still convey spatial information,鈥� says Benchenane. 鈥淭hat provides evidence that when you鈥檝e got activation of place cells during the consolidation of memories in sleep, you鈥檝e got consolidation of the spatial information.鈥�
Benchenane hopes that his technique could be developed to help alter people鈥檚 memories, perhaps of traumatic events (see 鈥�Now it鈥檚 our turn鈥�).
at the University of California, San Francisco, agrees. 鈥淚 think this is a really important step towards helping people with memory impairments or depression,鈥� he says. 鈥淚t is surprising to me how many neurological and psychiatric illnesses have something to do with memory, including schizophrenia and obsessive compulsive disorder.鈥�
鈥淚n principle, you could selectively change brain processing during sleep to soften memories or change their emotional content,鈥� he adds.
Journal reference: Nature Neuroscience, doi:
Now it鈥檚 our turn
It鈥檚 a familiar feat from films such as Inception and Total Recall, but will we ever really be able to plant a memory in someone else鈥檚 mind as they sleep?
Karim Benchenane at the Industrial Physics and Chemistry Higher Educational Institution in Paris, France, who implanted new memories into mice while they snoozed (see main story), hopes his technique can be developed to alter problematic memories in people. The idea is to attach good thoughts to bad memories, such as those that linger after traumatic experiences. 鈥淚f you can identify where in the brain a person is reactivating a phobia-associated experience, you might be able to create a positive association,鈥� he says.
You could probably use the same approach to alter a person鈥檚 memory to your own advantage.
Evidence suggests that single neurons can represent specific people in the brain 鈥� such cells have been termed 鈥淛ennifer Aniston cells鈥� after a woman in a study was found to have one brain cell that only fired in response to images of the actress (Nature, ). If you could identify a neuron that represents you in someone else鈥檚 brain and then stimulate areas of the brain that create a rewarding feeling every time that neuron fires, you might 鈥� in theory 鈥� be able to make that person like you more. 鈥淭he fact that you can do it during sleep is a bit worrying, in that it implies that you could make somebody want something even if they didn鈥檛 really,鈥� says at University College London.
It is much more difficult to create an entirely new memory from scratch. Benchenane鈥檚 team drew on the mice鈥檚 existing memories of space and altered them. 鈥淚t鈥檚 not like they have created a whole new space that the animal is exploring in its head,鈥� says at the University of California, San Francisco. 鈥淩eal experiences involve all of our senses and movement through space, and people, places and things,鈥� he says. 鈥淲e are nowhere near recreating that richness 鈥� what we can do is take advantage of it and modify it.鈥�
These modifications could be for better or worse, says Frank. 鈥淭here are a few ways of thinking about this 鈥� there鈥檚 the medical application, and there鈥檚 the more Orwellian application, where the government gets inside people鈥檚 heads and starts to control them,鈥� he says. 鈥淚t鈥檚 unbelievably hard to do any of this, so I鈥檓 not deeply worried about it, but it鈥檚 not impossible that it could happen.鈥�
This article appeared in print under the headline 鈥淔alse memories conjured in sleep鈥�
