Mice can recall artificial memories created during sleep once they’re awake, researchers report in Nature Neuroscience. The findings support a causal role between the firing of specialized neurons called place cells and the ability of these neurons to represent a particular location in space. The study shows that the emotional value of a particular [location] can be modified, and, what is most critical, is that this can happen in a subconscious, sleep state.
“This is the first example of memory manipulation during sleep,” said Buzsáki. “The ideas in this study support the probably 100-year-old idea that it is possible to put false memories in the brain.”
This linking of a specific place cell with rewarding stimulation resulted in the animals spending about five times more time in the location associated with the place cell activity compared to the time they spent in that location prior to the stimulation experiment.
When this link between the place cell and reward trigger occurred during sleep in a second set of mice, the animals were more likely to go directly to the location associated with that place cell upon waking.
“The animal developed a goal-directed strategy for the [location], as if the animal had a conscious recollection that there was a reward there,” explained Benchenane.
Hippocampal place cells assemblies are believed to support the cognitive map, and their reactivations during sleep are thought to be involved in spatial memory consolidation. By triggering intracranial rewarding stimulations by place cell spikes during sleep, we induced an explicit memory trace, leading to a goal-directed behavior toward the place field. This demonstrates that place cells’ activity during sleep still conveys relevant spatial information and that this activity is functionally significant for navigation.
a. Wake pairing protocol. The paradigm consisted in four sessions: 1) free exploration and place field identification, 2) basal exploration (PRE), 3) 15 minutes exploration with pairing where online detected spikes of a given place cell triggered MFB stimulation, and 4) test exploration (POST). In PRE and POST session a minimum of eight 60s—trials with changing starting point (gray circles) is performed. Analyses were carried on the first 4 trials where animal was actively exploring. b. Sleep pairing protocol. As in panel a, the sleep protocol is composed of the same four sessions except that spike—stimulation pairing is performed during 1h of sleep. This protocol was always followed by free exploration to erase the learning (extinction exploration) and a control wake pairing. c. Classical Place preference task. Mouse position was tracked in real time with a video camera during a session lasting 15 minutes. Rewarding stimulations were then delivered when mice reached a pre—established area, covering around 5% of the total surface of exploration. This stimulation phase was preceded by a basal exploration (PRE) and followed by a probe exploration (POST) as described in panel a. d. Method for the choice of the 4 starting points used in PRE and POST sessions of all place preference paradigms.
Pairing place cell to rewarding stimulation during wakefulness or sleep to create a place preference.
Karim Benchenane at the Industrial Physics and Chemistry Higher Educational Institution in Paris, France, who implanted new memories into mice while they snoozed, 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. “If you can identify where in the brain a person is reactivating a phobia-associated experience, you might be able to create a positive association,” he says.
SOURCES – Nature Neuroscience, The Scientist, New Scientist
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