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 implicates subcortical structures, especially the thalamus, in slow wave generation.

Purpose of the Study:

• To investigate the distinct mechanisms underlying single slow waves.
• To test the hypothesis of separate cortico-cortical and thalamo-cortical origins for slow waves.
• To explore the role of the thalamus in synchronizing slow waves.

Main Methods:

• Simultaneous electroencephalography (EEG) and functional magnetic resonance imaging (fMRI) were employed in healthy adults.
• fMRI data were used for spatial mapping of thalamic responses during slow waves.
• Slow waves were classified based on distinct patterns of thalamic activation.

Main Results:

• Two types of slow waves (C1 and C2) were identified based on thalamic fMRI responses.
• Type C1 waves showed an early thalamic signal increase, correlating with larger, synchronized waves, sleep spindles, and heightened arousal/autonomic activity.
• Type C2 waves exhibited an initial negative fMRI response, linked to smaller slow waves potentially arising from cortico-cortical synchronization during stable NREM sleep.

Conclusions:

• Distinct subtypes of slow waves exist, characterized by differential thalamic involvement.
• These findings suggest varying synchronization mechanisms, potentially cortico-cortical or thalamo-cortical, underlie different slow wave types.
• The study provides new insights into the neural basis of sleep oscillations and their regulation.