Partial synchronization as a model for unihemispheric sleep (Vol. 50, No. 5-6)

Human brains exhibit a slight structural asymmetry of their two hemispheres (see Figure). We have investigated the dynamical asymmetry arising from this natural structural difference in healthy human subjects, using a minimum model which elucidates the modalities of unihemispheric sleep in human brain, where one hemisphere sleeps while the other remains awake. In fact, this state is common among migratory birds and mammals like aquatic species.

By choosing appropriate coupling parameters in a network of FitzHugh-Nagumo oscillators with empirical structural connectivity, we have observed that our brain model exhibits spontaneous symmetry breaking and bistability, where each hemisphere may engage into either of two dynamical states, characterized by a relatively high and low degree of synchronization. However, a high degree of synchronization in one of the hemispheres always coincides with a low degree of synchronization in the other. This dynamical asymmetry can be even enhanced by tuning the inter-hemispheric coupling strength. These results are in accordance with the assumption that unihemispheric sleep requires a certain degree of inter-hemispheric separation.

The structural asymmetry in the brain allows for partial synchronization dynamics, which may be used to model unihemispheric sleep or explain the mechanism of the first-night effect in human sleep.

L. Ramlow, J. Sawicki, A. Zakharova, J. Hlinka, J. Ch. Claussen and E. Schöll, Partial synchronization in empirical brain networks as a model for unihemispheric sleep, EPL 126, 50007 (2019)
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