How we learn and forget while sleeping

Scientists have made a breakthrough in understanding how sleep controls what we remember and what we forget. It may all come down to the kind of brainwaves rippling through your grey matter as you snooze.

Catching the right number of zzz’s is already known to be important for laying down memories, hence the advisory that students swotting for a major test back up with a good night’s sleep.

Dreamland is also where memories can be selectively faded, something that prevents the mind’s finely tuned mechanics getting clogged with useless info.

But just how sleep can amp up the signal and dial down the noise isn’t clear. 

A team, led by neurologist Karunesh Ganguly at the University of California, San Francisco in the US, has set about fixing that, aided by mice with some very sophisticated hardware in their noggins.

Ganguly’s team put the mice to work learning how to get water to flow from a spout. With their minds.

That clever bit of conjuring was made possible with something called a brain-machine interface, a set of electrodes implanted in the mice’s brains that turned on the tap when it picked up a precise sequence of neural activity.

The water reward was, in effect, part of a biofeedback loop that taught the animals to switch on a tiny “ensemble” of brain cells.

Later, when the critters were getting shuteye after their labours, the researchers found those very same neurons were active all over again. It was the brain in maestro mode, consolidating the new learning as memory while they slept.  

But the researchers had done something else to those mice that would sabotage their carefully learned water routine.

They sent in a virus to deliver a light sensitive protein into their brains. With this kit in place, the scientists could turn brain cells on or off at will, using fibreoptics and a laser.

It’s called optogenetics and the team used it to seriously mess with the brainwaves of the sleeping mice.

It is a curious property of brain cells to fire in unison at a given frequency – these are the “brainwaves” picked up as regular spikes on the readout of an EEG machine. During sleep, you get delta waves at up to 4 Hz and a related beast called slow oscillations.

Ganguly’s team decided to interrupt each class of brainwave in their slumbering rodents. It was a manoeuvre with profound effects on the water task – good and bad.

“We were astonished to find that we could make learning better or worse by dampening these distinct types of brain waves during sleep,” says Ganguly.

Interrupting slow oscillations made the mice worse at getting water when they woke up. Conversely, blocking delta waves not only enhanced firing of the new learning circuit, but also boosted performance on the water task.

“Slow oscillations seem to be protecting new patterns of neural firing after learning, while delta waves tend to erase them and promote forgetting,” says Ganguly.

“We believe these two types of slow waves compete during sleep to determine whether new information is consolidated and stored, or else forgotten.”

The researchers say the findings could help understand how human brain-machine interfaces, which people use to move prosthetic limbs or operate a computer cursor with their thoughts, integrate into the brain.

“Sleep is truly driving profound changes in the brain,” says Ganguly. 

“Understanding these changes will be critical for brain integration of artificial interfaces and may one day allow us to modify neural circuits to aid in movement rehabilitation, such as after stroke, where previous studies have shown that sleep plays an important role in successful recovery”.

The research appears in the journal Cell.