Thalamic involvement defines distinct slow-wave subtypes in NREM sleep

Area of Science:

• Neuroscience
• Sleep Science
• Cognitive Neuroscience

Background:

• Slow waves (0.5-4 Hz) are characteristic of non-rapid-eye-movement (NREM) sleep.
• Traditionally, slow waves were attributed to neocortical circuits.
• Emerging evidence points to a significant role for subcortical structures, especially the thalamus, in slow wave generation and synchronization.

Purpose of the Study:

• To investigate the hypothesis that slow waves originate from distinct cortico-cortical and thalamo-cortical mechanisms.
• To differentiate slow wave subtypes based on their generation and synchronization processes.
• To explore the role of the thalamus in initiating and synchronizing slow waves.

Main Methods:

• Simultaneous electroencephalography (EEG) and functional magnetic resonance imaging (fMRI) were employed in healthy adults.
• Thalamic fMRI responses were spatially mapped to identify distinct slow-wave associated clusters.
• Analysis focused on the timing and nature of thalamic signals relative to slow wave events.

Main Results:

• Two distinct slow-wave associated clusters were identified based on thalamic fMRI responses.
• Cluster 1 (C1) showed an early thalamic fMRI signal increase, linked to large, synchronized waves, sleep spindles, and heightened arousal/autonomic activity.
• Cluster 2 (C2) exhibited an initial negative fMRI response, associated with smaller slow waves potentially arising from cortico-cortical synchronization, often during low-fragility sleep phases.

Conclusions:

• Slow waves represent distinct subtypes with varying degrees of thalamic involvement.
• Different synchronization mechanisms may underlie these identified slow wave subtypes.
• This research refines our understanding of the neural basis of NREM sleep oscillations.