Bundled white matter fibers in the corpus callosum that connect the "right brain" to the "left brain" allow slow brain waves to travel back and forth between both cerebral hemispheres during slow-wave sleep (SWS), according to a new study involving "split-brain" patients without a fully functioning corpus callosum. These findings (Avvenuti et al., 2020) were published on June 15 in the Journal of Neuroscience.
Slow-wave sleep is the deepest phase of dreamless, non-rapid eye movement (NREM) sleep. On an electroencephalogram (EEG) recording, SWS is characterized by "slow" delta brain waves (1-4 Hz).
This research suggests that having a well-connected corpus callosum is essential for the cross-hemispheric dispersion of slow waves during NREM sleep. According to the authors, this is the first time that sleep researchers have identified that non-REM sleep "slow waves" travel between brain hemispheres via so-called "anatomical highways" in the corpus callosum.
For this high-density EEG sleep study of split-brain patients, researchers from the IMT School for Advanced Studies Lucca in Italy enlisted a rare population of "corpus callosotomy" neurosurgery patients who had undergone a procedure that separates the white matter fiber tracts that connect the cerebral hemispheres via the corpus callosum.
Each of the five split-brain patients in this sleep study had undergone a total callosotomy to treat severe, drug-resistant epilepsy. The researchers also enlisted a cohort of three non-callosotomized neurological patients and a sample of 24 healthy adults to serve as control groups.
In split-brain patients, the researchers found that sleep slow waves stayed in their brain hemisphere of origin and remained confined to either the left or right cerebral hemisphere during NREM deep sleep. "Our findings clearly indicate that sleep slow waves propagate through anatomical pathways connecting the origin site to distant brain areas," first author Giulia Avvenuti said in a news release.
"The slow waves of sleep are known to have a key role in the restorative functions of sleep, but they also seem to be important for the consolidation of recently acquired memories," co-senior author Giulio Bernardi added. "Their synchronization within particular connected networks of brain areas could be especially important for the processing of novel memories."
These results demonstrate the importance of the corpus callosum in the cross-hemispheric travel of sleep slow waves but also show that each cerebral hemisphere can independently generate NREM sleep slow waves. Interestingly, the researchers also discovered that in both callosotomized and healthy adults, sleep slow waves tend to originate more frequently in the "right brain" than in the "left brain."
Giulia Avvenuti, Giacomo Handjaras, Monica Betta, Jacinthe Cataldi, Laura Sophie Imperatori, Simona Lattanzi, Brady A. Riedner, Pietro Pietrini, Emiliano Ricciardi, Giulio Tononi, Francesca Siclari, Gabriele Polonara, Mara Fabri, Mauro Silvestrini, Michele Bellesi and Giulio Bernardi. "Integrity of Corpus Callosum Is Essential for the Cross-Hemispheric Propagation of Sleep Slow Waves: A High-Density EEG Study in Split-Brain Patients." Journal of Neuroscience (First published: June 15, 2020) DOI: 10.1523/JNEUROSCI.2571-19.2020