Strengthening activity of some brainwaves in rats’ while they sleep improves their performance on a memory test. If we can boost these in people, it could lead to new treatments for dementia and other memory impairments.
Memories are encoded in unique patterns of activity between neurons. Each time we learn or experience something new, a subset of neurons forms connections. These connections then strengthen while we sleep, reinforcing the memory. This process is known as memory consolidation.
Previous research has shown that memory consolidation relies on two brain regions: the hippocampus, which stores short-term memories, and the cortex, which stores long-term memories. suggests the hippocampus transfers short-term memories to the cortex during sleep using high-frequency brainwaves known as sharp wave ripples.
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To validate this, at Cornell University in New York and his colleagues disrupted communication between the hippocampus and cortex of 10 rats while they slept.
First, the researchers trained the rodents to find sugar water in a maze. Then, while the rats slept, they used optogenetics – a technique that switches specific cells on and off with light – to deactivate neurons in each animal’s cortex whenever its hippocampus produced sharp wave ripples.
The next day, the rats performed no better on the maze than they had the day before, suggesting they had little to no memory of it. “If we prevent the cortex from listening to the hippocampal messages during sleep, that impairs [memory] consolidation,” says Fernandez-Ruiz, who presented these findings at a meeting of the Society for Neuroscience in Chicago on 8 October. “But this isn’t only a one-way process, as has been traditionally believed,” he says.
The hippocampus isn’t the only region of the brain that produces brainwaves in sleep – the cortex does, too. Researchers showed that switching off hippocampal cells in the presence of these cortical brainwaves impaired the rats’ performance on memory tests the following day. “We found that there is a dialogue between these two structures,” says Fernandez-Ruiz. “If we prevent the hippocampus from listening to the cortex, it will prevent rats from remembering what they had just done.”
They also found that the process of reinforcing memories can be enhanced. The researchers placed 10 different rats alone in a cage with two new objects. After about 5 minutes, they were returned to their home cages. The scientists then used optogenetics to stimulate hippocampal neurons in the animals’ brains while they slept to strengthen sharp wave ripples.
The following day, the researchers returned the rats to the cage with two objects, but this time, one of the objects had been moved. If the rodents realised the object was moved –  which requires remembering its initial position – they will spend more time exploring it out of curiosity, says Fernandez Ruiz.
Compared with a control group of 10 rats, those with boosted brainwaves spent 40 per cent more time, on average, near the moved object, indicating an improvement in memory consolidation.
By better understanding how the brain reinforces memories in sleep, we may be able to improve the process of memory formation, says at Harvard University, who wasn’t involved in the study. This could lead to new treatments for Alzheimer’s disease, as people with the condition tend to spend less time in sleep stages important for memory consolidation, she says.
However, it isn’t feasible to use optogenetics to boost brainwaves in people, says Fernandez-Ruiz, as the technique is invasive and involves gene editing. He and his colleagues have shown similar effects in rodents by placing electrodes on their scalp, but it is unclear if this would be effective in humans, he says.
