Although cortically generated, slow waves are thought to emerge from the dynamic interplay between thalamic nuclei and the cerebral cortex ( Murphy et al., 2009 Crunelli et al., 2015). These epiphenomena reflect slow oscillations that occur at a cellular level where intense and synchronized neuronal firing alternates with a period of silent state. When the brain departs from wakefulness to enter into deep sleep, it shifts from a state of desynchronized EEG activity to an avalanche of spontaneous slow (75 μV) waves. Our results showed that the relationship between white matter and the brain’s ability to synchronize during sleep is neither linear nor simple. Moreover, our observations challenge the current line of hypotheses that white matter microstructure deterioration reduces cerebral synchrony during sleep. Our results suggest that, contrary to previous observations in healthy controls, white matter damage does not prevent the expected high cerebral synchrony during sleep. These associations between white matter damage and sleep were found only in our traumatic brain injured participants, with no such correlation in controls. More specifically, higher white matter damage was associated with higher slow-wave activity power, as well as with more severe complaints of cognitive fatigue. The same pattern of associations with white matter damage was also observed with markers of high homeostatic sleep pressure. Contrary to our hypotheses, we found that greater white matter damage mainly over the frontal and temporal brain regions was strongly correlated with a pattern of higher neuronal synchrony characterized by slow waves of larger amplitudes and steeper negative-to-positive slopes during non-rapid eye movement sleep.
Correlation analyses were performed in traumatic brain injury and control participants separately, with age as a covariate. We measured the following slow wave characteristics for all slow waves detected in N2 and N3 sleep stages: peak-to-peak amplitude, negative-to-positive slope, negative and positive phase durations, oscillation frequency, and slow wave density. fractional anisotropy as well as mean, axial and radial diffusivities) to characterize voxel-wise white matter damage. We used MRI and diffusion tensor imaging metrics (e.g. Here, we answer this question by testing 23 patients with various levels of white matter damage secondary to moderate to severe traumatic brain injuries (ages 18–56 17 males, six females, 11–39 months post-injury) and compared them to 27 healthy subjects of similar age and sex. However, whether a brain can properly synchronize and produce a restorative sleep when it undergoes massive and widespread white matter damage is unknown. The restorative function of sleep partly relies on its ability to deeply synchronize cerebral networks to create large slow oscillations observable with EEG.
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