Sawtooth Wave Oscillations During REM Sleep

Post by Cody Walters

What’s the science?

Sawtooth waves (STWs) are jagged oscillations seen in EEG recordings prior to bursts of rapid eye movement (REM) during sleep. Although REM sleep has been the focus of much research, little is known about the cortical origin and functional significance of STWs. This week in The Journal of Neuroscience, Frauscher et al. performed the first ever intracranial study of STWs in human subjects.

How did they do it?

The authors analyzed data from 26 epileptic human patients that underwent preoperative stereoelectroencephalography (SEEG) recordings to localize epileptogenic brain tissue. In addition to SEEG recordings, the authors had access to scalp EEG and polysomnography data.

What did they find?

The authors identified that STWs occur in a variety of brain structures including the parietal, lateral, and medial areas; the anterior insula; the lateral and orbital frontal cortices; and mesiotemporal structures. They then quantified the spectral power (by decomposing the SEEG signal into a sum of sine waves with varying frequencies and amplitudes) in a variety of brain regions during STWs. There was an increase in both low (2-4 Hz) and high (20-240 Hz) frequency power (i.e., the amplitude of the sine wave in those frequency ranges significantly contributed to the overall SEEG signal) during STWs in most brain regions.

Ripples are oscillations in the local field potential that occur in the high frequency range (i.e. 80-240 Hz ) and are known to play a role in memory consolidation and replay. The authors found that there was a significant increase in the ripple rate in a variety of brain structures (e.g., the angular gyrus, posterior cingulate gyrus, temporal pole, superior temporal gyrus and mesiotemporal structures) during STWs. Furthermore, the authors found that STWs were temporally and spatially highly variable both within and between brain regions. These data suggest that STW may synchronize the re-activation of activity to aid in complex memory consolidation.

What’s the impact?

This study is the first intracranial investigation of the electrophysiological correlates of sawtooth waves in human subjects. The authors found that (1) STWs occured in various brain regions, (2) STWs co-occurred with high frequency ripple events, and (3) STWs were both temporally and spatially variable. These data suggest that STWs may play an important role in coordinating the brain activity to aid memory consolidation during REM sleep. Further, this study sheds light on a largely unexplored aspect of REM sleep.

Frauscher et al. (2020). REM sleep sawtooth waves are associated with widespread cortical activations. The Journal of Neuroscience. Access the publication here.