Rodent recall: false but happy memories implanted in sleeping mice

Scientists have succeeded in creating false but happy memories in mice, in the first demonstration of memory manipulation during sleep.

In the study, positive feelings about a particular place were artificially written into the animal’s memory, which caused them to seek out that place in search of a reward when they woke up.

The discovery that emotional memory can be readily manipulated has echoes of the the film, Eternal Sunshine of the Spotless Mind, in which Jim Carey’s character has every recollection about his failed relationship wiped clean using a fictional mind-reading technology.

The scientists say that the findings could pave the way for new treatments that would allow patients to overcome depression or deeply entrenched painful memories.

Karim Benchenane, the neuroscientist who led the research at the CNRS in Paris, said: “The idea is to use this as a tool for post-traumatic stress disorder.”

Steve Ramirez, a neuroscientist at Massachusetts Institute of Technology who is also working on memory manipulation, described the research as a “technical tour-de-force”. He said: “It demonstrates a little bit of mastery over how the brain works. “In future it should be possible to dampen the punch of traumatic memories in people.”

In the study, scientists implanted two electrodes in the brains of mice – one in a region of the hippocampus, the brain’s memory centre, involved in mapping locations and the second one in the brain’s reward centre.

In both mice and humans, our environment is physically mapped out in the brain by so-called “place cells”, which light up like coordinates on a grid as we move through space. The scientists picked a particular place cell neuron and observed which location it related to in real-life as the mouse roamed around a large exploration area.

During sleep, mice typically replay these sequences of place cell activity - effectively their memory replaying where they have been during waking hours.

The scientists rigged up the electrodes, such that during sleep, whenever the target place cell became active it triggered the delivery of a reward stimulation in the second electrode.

Although the memory of the place was initially neutral, during its sleep, the mouse learned to associate it with something positive happening.

“When they woke up the mouse went straight to that place as though it was waiting for a reward,” said Benchenane.

The study demonstrates what we subjectively know to be true — that the factual content of our memories and the emotional content are stored in different brain centres and can be changed independently.

Benchenane believes the findings could be the basis for a non-invasive tool for memory manipulation in humans. Rather than electrodes, functional MRI scans could be used to identify when a person is replaying a specific memory during sleep, he suggested.

However, the authors said they would be cautious about rushing to try and test this in PTSD patients. “It might be extremely dangerous to get a reward for something fearful,” he said.

Ramirez agreed that in future, the findings could have clinical applications. “The big thing technologically is we don’t have a good non-invasive way of manipulating brain activity. One day it should be possible though,” he said.