Which Neurons Go to Sleep First in Humans? fMRI Can Tell

Amazing stuff! Towards better sleep and less insomnia!

"In the last decades, science has taught us that the mammalian brain isn’t always entirely awake or asleep. Dolphins can swim with one hemisphere asleep while the other is alert, and some neurons in sleep-deprived rats can “switch off” while the animals are still awake. In humans, this so-called “local sleep,” in which specific neuronal populations take a nap while the rest of the brain is awake, has been more challenging to study, since the invasive methods used to track it in other mammals cannot be used on people.

A new study published July 21 in PNAS seems to have overcome this challenge. By simultaneously mapping human brain signals measured with two different methods (one with good temporal resolution and the other with good spatial resolution), the team pinpointed the waking or sleeping state of neuronal populations at the local level. The achievement made it possible to identify which brain regions are the first to fall asleep and which are the first to wake up, and experts say it promises to be a valuable tool for studying sleep in humans. ..."

"Significance
The conventional wisdom that sleep is a global state, affecting the whole brain uniformly and simultaneously, was overturned by the discovery of local sleep, where individual neuronal populations were found to be asleep and the rest of the brain awake. However, due to the difficulty of monitoring local neuronal states in humans, our understanding of local sleep remains limited. Using simultaneous functional MRI (fMRI) and electroencephalography, we find that the oscillations of brain hemodynamic activity provide signatures of sleep at a local neuronal population level. We show that the fMRI signatures of sleep can be employed to monitor local neuronal states and investigate which brain regions are the first to fall asleep or wake up at wake–sleep transitions."

From the abstract:

"Sleep can be distinguished from wake by changes in brain electrical activity, typically assessed using electroencephalography (EEG). The hallmark of nonrapid-eye-movement (NREM) sleep is the shift from high-frequency, low-amplitude wake EEG to low-frequency, high-amplitude sleep EEG dominated by spindles and slow waves. Here we identified signatures of sleep in brain hemodynamic activity, using simultaneous functional MRI (fMRI) and EEG. We found that, at the transition from wake to sleep, fMRI blood oxygen level–dependent (BOLD) activity evolved from a mixed-frequency pattern to one dominated by two distinct oscillations: a low-frequency (<0.1 Hz) oscillation prominent in light sleep and correlated with the occurrence of spindles, and a high-frequency oscillation (>0.1 Hz) prominent in deep sleep and correlated with the occurrence of slow waves. The two oscillations were both detectable across the brain but exhibited distinct spatiotemporal patterns. During the falling-asleep process, the low-frequency oscillation first appeared in the thalamus, then the posterior cortex, and lastly the frontal cortex, while the high-frequency oscillation first appeared in the midbrain, then the frontal cortex, and lastly the posterior cortex. During the waking-up process, both oscillations disappeared first from the thalamus, then the frontal cortex, and lastly the posterior cortex. The BOLD oscillations provide local signatures of spindle and slow wave activity. They may be employed to monitor the regional occurrence of sleep or wakefulness, track which regions are the first to fall asleep or wake up at the wake–sleep transitions, and investigate local homeostatic sleep processes."

Which Neurons Go to Sleep First in Humans? fMRI Can Tell | The Scientist Magazine® By linking blood flow patterns to bioelectric signals in the brains of sleeping volunteers, scientists are studying the order in which brain regions fall asleep and wake up.

fMRI spectral signatures of sleep (open access)