Two distinct synchronization processes in the transition to sleep: a high-density electroencephalographic study.

Area of Science:

• Neuroscience
• Sleep Science

Background:

• Understanding the falling-asleep process is crucial for sleep research.
• The characteristics of electroencephalographic (EEG) signals like slow waves and spindles evolve during sleep onset.

Purpose of the Study:

• To investigate the dynamic changes in slow waves and spindles during the transition to sleep.
• To characterize the temporal and spatial evolution of these EEG features.

Main Methods:

• High-density electroencephalography (EEG) recordings in six healthy participants.
• Serial awakenings at 15-30 minute intervals during overnight recordings.
• Analysis of 141 falling-asleep periods at both scalp and source levels.

Main Results:

• Slow wave number increased progressively, while amplitude showed an initial rapid rise followed by a linear decrease.
• Early slow waves were large, steep, widespread, frontomedially predominant, and originated from sensorimotor/parietal cortex.
• Later slow waves were smaller, less steep, more localized, with broader origins; spindles became more numerous and slower.

Conclusions:

• Evidence suggests two distinct types of slow waves with different temporal courses and cortical origins during sleep onset.
• Hypothesized two synchronization processes: a "bottom-up" subcorticocortical process (Type I) and a "horizontal" corticocortical process (Type II).
• The dissociation of these processes may be relevant for understanding sleep disorders and experimental manipulations.