Sleep is a nightmare for neuroscientists but a new study using electrodes implanted deep within the brains of people going about their daily lives has revealed that the brain falls asleep from the inside out, contrary to what was expected.
Most neuropsychology studies require people to complete tasks while the brain is being monitored and the technologies that allow passive recording either only measure activity on the brain surface (EEG, MEG) or are too uncomfortable to measure realistic sleep (fMRI, PET). This is one of the reasons human sleep has been difficult to study and why we still understand little about it.
A new study just published online in the Proceedings of the National Academy of Sciences used the innovative technique of recording from semi-permanent electrodes implanted in the brains of 13 people undergoing assessment for difficult-to-treat epilepsy. These electrodes stay in for several weeks, meaning the researchers had access to brain activity as people continued their lives and, of course, as they drifted off to sleep.
Certain types of epilepsy don’t respond to normal treatment and neurosurgery to remove a small part of the brain that triggers the seizures is known to be an effective treatment in many cases. However, this is only feasible when it’s possible to locate where the seizures originate.
In rarer cases still, a standard EEG or brief surgical test doesn’t give a good idea of where this might be, so surgeons can insert depth electrodes into the most likely areas. These remain in place and record any unusual activity directly from locations across the brain.
The researchers, led by neuroscientist Michel Magnin from the University of Lyon, asked the patients if they could also use this data to help understand what happened during sleep onset.
This is surprising because the thalamus has traditionally been considered a structure that regulates alertness and ‘relays’ information to the rest of the brain from the body and the spinal cord.
It was often assumed that it would ‘shut down’ the cortex first, because this is often considered to be where our ‘higher’ conscious functions like abstract thought and complex perception lie, while continuing with its minimal vigilance functions. A bit like a neural ‘standby’ setting.
Instead, what seems to happen is that the thalamus ‘disconnects’ itself and leaves the cortex freewheeling before it finally settles down into inactivity.
Indeed, freewheeling is, perhaps, a good description here. The researchers found lots of uneven activity in the upper brain areas as they were left to drift off.
Interestingly, sleep onset is one of the times when we are most likely to experience hallucinations. In fact, they are so common as to have been given their own name – hypnagogic hallucinations – while this drifting off period is known as hypnagogia.
Although they didn’t specifically ask about the whimsical thoughts and unusual perceptions that typically occur in this state, the researchers speculate that this pattern of freewheeling close-down might explain why hallucinations are so common at this time.