Sleep State Influences Early Sound Encoding at Cortical But Not Subcortical Levels.

Area of Science:

• Neuroscience
• Sleep Science
• Auditory Neuroscience

Background:

• The brain must balance memory consolidation during sleep with responsiveness to external stimuli.
• Auditory processing changes with sleep depth, but early sound representation changes are unclear.
• The frequency-following response (FFR) reflects neural encoding of sound periodicity across the auditory system.

Purpose of the Study:

• To investigate how sleep depth and microarchitecture affect auditory encoding.
• To evaluate sleep-dependent changes in neural representation strength throughout the auditory neuraxis.
• To determine if neural events like slow waves and sleep spindles impact acoustic processing during sleep.

Main Methods:

• Recorded frequency-following responses (FFRs) during a 2.5-hour nap in 14 healthy adults.
• Utilized magnetoencephalography to resolve FFR neural sources across the auditory system.
• Analyzed FFR strength in relation to sleep stages, slow waves, and sleep spindles.

Main Results:

• FFR strength was maintained in subcortical auditory nuclei across non-rapid eye movement sleep stages.
• FFR strength decreased in the auditory cortex during deeper sleep stages.
• Reduced thalamus-cortex communication, not slow waves or spindles, correlated with changes in FFR strength.

Conclusions:

• Auditory processing is differentially affected by sleep depth across the auditory hierarchy.
• Subcortical auditory encoding is preserved during sleep, while cortical encoding is reduced.
• This differentiation may allow the brain to balance environmental monitoring with sleep's restorative functions.