When falling asleep, the thalamus first shuts down its activity. Photo: Alessandro Zangrilli, public domain
Reading aloud Falling asleep in the brain does not happen at the same time everywhere, but gradually the light goes out. The thalamus starts with this: The brain area, often referred to as the "gate to consciousness", already shuts down its activity minutes before the cerebral cortex, the seat of consciousness, showed French researchers. That might explain why so many people see and hear things that are not there just before falling asleep, the researchers believe? after all, the thalamus serves as a kind of filter that assesses incoming signals and lets only the most important of them pass into consciousness. If this filter is switched off, unusual connections and thus unusual images can arise in the cerebral cortex. By the way, when you wake up, the situation looks different: Thalamus and cerebral cortex work perfectly synchronized. Normally, activities in the thalamus and cerebral cortex are closely linked. For some time, however, there has been increasing evidence that this coupling does not last while sleeping. However, a more detailed examination is difficult, as a normal brain current measurement with scalp-mounted electrodes does not provide enough detail to separate the activities in the brain regions exactly.

For this reason, Magnin from the University of Lyon 1 and his colleagues now decided to investigate the processes of falling asleep in 13 very special subjects: They were used to treat epilepsy disease electrodes in the brain, with the help of directly activities in the appropriate regions. The evaluation of the data showed an unexpectedly clear result: in over 93 percent of the measurements, the activity in the thalamus had initially dropped and only then, with an average delay of more than nine minutes, in the cerebral cortex. In addition, this drop in activity not only set in later, it was also slower.

Apparently, the thalamus is sent to sleep earlier than the cerebral cortex by the sleep control centers hypothalamus and brainstem, the scientists write. In this phase, the consciousness can then move freely, so to speak, which leads to the misinterpretation of certain signals and thus to the frequently observed hallucinations. Also, the feeling that it took much longer to fall asleep than it actually was, was probably due to this decoupling. The question remains, how this effect comes about. It is conceivable that the cerebral cortex reacts lazily to the sleep commands than the thalamus and therefore shuts off later. Alternatively, it could also be an active process that performs a previously unknown function.

Michel Magnin (University of Lyon 1) et al .: PNAS, online pre-publication, doi: 10.1073 / pnas.0909710107 ddp / science.de - Ilka Lehnen-Beyel advertisement

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