Stages of
deep sleep can significantly change our consciousness, just as it does in a
coma or under anesthesia. Scientists have hypothesized that brain activity
declines when we sleep, using research conducted with electroencephalography
(EEG), a process that uses electrodes placed along a patient’s scalp to measure
brain activity.
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 (MRI.) Dimitri Van De Ville, lab lead,
said, “MRI scans measure neural activity by providing important information in
addition to EEGs.” Tarun used EEG to identify when study patients fell asleep,
pinpointing the individual sleep stages. The MRI images were later used to produce
spatial maps of neural activity, specifying brain states.
Deep sleep
is reasonably challenging to achieve while undergoing an MRI, as the machines
are quite noisy. Despite the hurdle, Tarun was able to leverage simultaneous
MRI and EEG data from roughly thirty people. “Two hours is a relatively long
time, meaning we were able to obtain a set of rare, reliable data,” she said.
“MRIs carried out while a patient is performing a cognitive task usually last
around 10 to 30 minutes.”
The data
Tarun collected was surprising. “We calculated exactly how many times networks
made up of different parts of the brain became active during each stage of
sleep. 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
said, “What really surprised us in all of 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.”
Neural
networks might be linked to our introspection process, memory, and spontaneous
thoughts, all associated with consciousness. “We saw that the network between
the anterior and posterior regions broke down, and this became increasingly
pronounced with increasing sleep depth. A similar breakdown in neural networks
was also observed in the cerebellum, which is typically associated with motor
control.” At this point, the researchers aren’t sure why this happens; their
findings are a novel step toward a better understanding of our sleep
consciousness.
“Our
findings show that consciousness is the result of interactions between
different brain regions, and not in localized brain activity. 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,” said Tarun.
For further reading, find the original article from EPFL.
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