Our state of consciousness changes significantly during stages of deep sleep, just as it does in a coma or under general anesthesia. Scientists have long believed – but couldn’t be certain – that brain activity declines when we sleep.
Most research on sleep is
conducted using electroencephalography (EEG), a method that entails measuring
brain activity through electrodes placed along a patient’s scalp. However,
Anjali Tarun, a doctoral assistant at EPFL’s Medical Image Processing
Laboratory within the School of Engineering, decided to investigate brain
activity during sleep using magnetic resonance imaging, or MRI. According to
Dimitri Van De Ville, who heads the lab, “MRI scans measure neural activity by
detecting the hemodynamic response of structures throughout the brain, thereby
providing important information in addition to EEGs.”
During these experiments,
Tarun relied upon EEG to identify when the study participants had fallen asleep
and pinpoint the different stages of sleep. Then she examined the MRI images to
generate spatial maps of neural activity and determine different brain states.
Difficult
data to obtain
The only catch was that it
wasn’t easy to perform brain MRIs on participants while they were sleeping. The
machines are very noisy, making it hard for participants to reach a state of
deep sleep. But working with Prof. Sophie Schwartz at the University of Geneva
and Prof. Nikolai Axmacher at Ruhr-Universität Bochum, Tarun could leverage
simultaneous MRI and EEG data from around thirty people.
The brain-activity data were
covered a period of nearly two hours while participants were sleeping in an MRI
machine. “Two hours is a relatively long time, meaning we were able to obtain a
set of rare, reliable data,” says Tarun. “MRIs carried out while a patient is
performing a cognitive task usually last around 10-30 minutes.”
Brain
activity during sleep
After checking, analyzing and
comparing all the data, what Tarun found was surprising. “We calculated exactly
how many times networks made up of different parts of the brain became active
during each stage of sleep,” she says.
“We discovered that during
light stages of sleep – that is, between when you fall asleep and when you
enter a state of deep sleep – overall brain activity decreases. But
communication among different parts of the brain becomes much more dynamic. We
think that’s due to the instability of brain states during this phase.”
Van De Ville adds: “What
really surprised us in all this was the resulting paradox. During the
transition phase from light to deep sleep, local brain activity increased and
mutual interaction decreased. This indicates the inability of brain networks to
synchronize.”
The
role of default-mode networks and the cerebellum
Consciousness is generally
associated with neural networks that may be linked to our introspection
processes, episodic memory and spontaneous thought. “We saw that the network
between the anterior and posterior regions broke down, and this became
increasingly pronounced with increasing sleep depth,” says Van De Ville. “A
similar breakdown in neural networks was also observed in the cerebellum, which
is typically associated with motor control.”
For now, the scientists don’t
know exactly why this happens. But their findings are a first step towards a
better understanding of our state of consciousness while we sleep.
“Our findings show that
consciousness is the result of interactions between different brain regions,
and not in localized brain activity,” says Tarun. “By studying how our state of
consciousness is altered during different stages of sleep, and what that means
in terms of brain network activity, we can better understand and account for
the wide range of brain functions that characterize us as human beings.”
Source: https://actu.epfl.ch/news/mri-helps-unravel-the-mysteries-of-sleep/
Source: MRI
Helps Unravel the Mysteries of Sleep – Scents of Science (myfusimotors.com)
No comments:
Post a Comment