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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