Brainwave activity changes dramatically across the different stages of sleep. It was long believed that brain activity declines when we sleep but was not certain.
Studies on sleep were often conducted using electroencephalography (EEG), which entails measuring brain activity through electrodes placed along a patient’s scalp. A new study by EPFL used MRI to investigate brain activity during sleep.
The result was unexpected: the study suggests that our brain tends to stay more active during sleep.
Dimitri Van De Ville said, “MRI scans measure neural activity by detecting the hemodynamic response of structures throughout the brain, thereby providing important information in addition to EEGs.”
During the experiments, scientists used EEG to identify when the study participants had fallen asleep and pinpoint the different sleep stages. Then by examining MRI images, scientists generated spatial maps of neural activity and determined different brain states.
It was a challenging task 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.
In collaboration with Prof. Sophie Schwartz at the University of Geneva and Prof. Nikolai Axmacher at Ruhr-Universität Bochum, scientists leveraged simultaneous MRI and EEG data from around thirty people. The brain-activity data were covered for nearly two hours while participants were sleeping in an MRI machine.
Anjali Tarun, a doctoral assistant at EPFL’s Medical Image Processing Laboratory within the School of Engineering, said, “Two hours is a relatively long time, meaning we were able to obtain a set of rare, reliable data. MRIs carried out while a patient is performing a cognitive task usually last around 10–30 minutes.”
“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 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.”
“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.”
“A similar breakdown in neural networks was also observed in the cerebellum, which is typically associated with motor control.”
Tarun said, “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.”
- Anjali Tarun et al. NREM sleep stages specifically alter dynamical integration of large-scale brain networks. DOI: 10.1016/j.isci.2020.101923