Previous studies have found that the brains of mammals contain an internal compass comprising head-direction cells whose activity pattern changes when the head points in a particular direction. Now, a new study of mice shows that these cells are just as active during sleep.
In the journal Nature Neuroscience, researchers at NYU Langone Medical Center in New York, NY, describe how they found that head-direction cells in mice are as electrically active during deep sleep as when the animals are awake.
Another surprising finding was that as the mice moved their heads during sleep, the head direction neurons showed the same firing patterns as when they received vision and balance cues in the awake state.
The researchers say it is as if the “internal compass” in the animals’ brains continues to code for the “virtual” direction of their gaze during sleep.
The team believes the discovery could lead to new treatments for brain and nerve disorders like Alzheimer’s disease, where the navigation system is one of the first to deteriorate.
Gyorgy Buzsaki, senior investigator and Biggs Professor of Neural Sciences at NYU Langone and its Neuroscience Institute, says:
“We have long known that the brain is at work during sleep. But now we know how it is working in one of the seemingly simpler senses – head orientation – or our sense of where we look at in any given space.”
He explains that obtaining a sense of direction is an essential part of the brain’s navigation system and it can reset our “internal compass” and maps in an instant, something we experience, for example, when we re-orient ourselves after emerging from the subway.
For their study, Prof. Buzsaki and colleagues compared the activity of head direction neurons in the antero-dorsal thalmic nucleus and postsibiculum regions of the brains of mice during various wake and sleep states.
They spent 2 years filming the head movements of the mice and recording the activity of head direction neurons in the two brain regions as they slept and as they navigated various environments.
They found that during rapid eye movement (REM) sleep, the firing activity of the head direction neurons in and between the two brain areas – the equivalent of the compass needle – moved at the same speed as it does during wakefulness.
In humans, REM is known to be a stage of sleep when intense dreaming occurs and during which electrical activity is virtually indistinguishable from when we are awake.
The researchers also found that during slow-wave periods of sleep – often referred to as deep sleep – the movement of the firing activity showed a 10-fold increase in acceleration, as if the mice were turning their heads 10 times faster than when they were awake.
Prof. Buzsaki says discovering that head direction neurons coordinate their activity during sleep suggests something is going on in the brain that substitutes for shifts in the animal’s gaze as it navigates while awake. The brain is actively exploring and coordinating even when it is disengaged from the environment.
He believes the findings confirm his view that mammals’ brains actively pursue sensory inputs – they are not waiting passively to receive them. The active sense of head directionality they observed in the mice as they slept is a good example of this.
Lead author Dr. Adrien Peyrache, a postdoctoral fellow in Buzsaki’s lab, says:
“The coordinated activity during the majority of sleep likely represents a consolidation of places, events and times, a sort of navigational backup system in the brain, during which the brain stores a map to memory.”
The team plans to find out if similar neuron activity occurs during more complex behaviors in other parts of the mouse brain. They also want to find out if it is possible to electrically control and predict head direction and navigation.
The study was funded by grants and fellowships from the National Institutes of Health, the National Science Foundation, the Human Frontier Science Program and the European Molecular Biology Organization.
In December 2014, Medical News Today also reported a study that showed our brains have a homing signal that points us in the right direction and continually updates itself as we move through our environment.