Abrupt and Gradual Changes in Neuronal Processing upon Falling Asleep and Awakening

Area of Science:

• Neuroscience
• Sleep Science
• Auditory Cortex Research

Background:

• Neural processes during sleep onset are not fully understood.
• Cortical activity, including spontaneous and sensory-evoked neural activity, differs between wakefulness and sleep.
• Specific changes like increased silent periods and population synchrony in the auditory cortex during sleep are known, but their timing around transitions is unclear.

Purpose of the Study:

• To investigate whether changes in neural activity around sleep onset and awakening occur abruptly or gradually.
• To characterize the temporal dynamics of stimulus-induced silent periods and population synchrony in the auditory cortex during vigilance state transitions.
• To correlate neural activity changes with electroencephalography (EEG) measures like K-complexes and slow-wave activity.

Main Methods:

• Recorded spontaneous and sound-evoked neuronal spiking activity in the rat auditory cortex.
• Utilized polysomnography to monitor sleep-wake states.
• Analyzed thousands of sleep onset and awakening episodes in male rats.
• Examined EEG auditory-evoked potentials and ongoing EEG slow-wave activity.

Main Results:

• Stimulus-induced neuronal silent periods (OFF periods) increased abruptly within seconds of sleep onset.
• Neuronal population synchrony gradually increased over tens of seconds.
• EEG auditory-evoked potentials correlated with postonset neuronal firing, while EEG slow-wave activity correlated with population synchrony.
• Opposite temporal dynamics were observed during awakenings.

Conclusions:

• Stimulus-induced neuronal silence is a key feature that changes abruptly around transitions between vigilance states (wakefulness, sleep).
• These abrupt changes likely reflect neuronal bistability and are observable in EEG-evoked potentials.
• Perturbing the nervous system provides valuable insights into state transitions, potentially aiding the development of noninvasive sleep monitoring tools.