Brain Activity and Changes in Consciousness During General Anesthesia

Participants in this arm will receive propofol. In young and older healthy volunteers, propofol will be administered by target-controlled infusion, with the plasma target concentration increased by 0.5 μg/mL every 5 minutes up to a maximum of 5 μg/mL to induce loss of responsiveness. EEG will be recorded during wakefulness, anesthetic induction, loss of responsiveness, and recovery. Pediatric surgical participants in this group will undergo EEG recording during anesthesia in the operating room.

Propofol will be administered by target-controlled infusion in young and older healthy volunteers. The plasma target concentration will be increased by 0.5 μg/mL every 5 minutes, up to a maximum of 5 μg/mL, until protocol-defined loss of responsiveness is achieved. Consciousness will be assessed every minute using the Modified Observer's Assessment of Alertness/Sedation (MOAA/S) scale, and EEG will be recorded throughout anesthetic induction and recovery.

Participants in this arm will receive remimazolam. In young and older healthy volunteers, remimazolam will be administered intravenously at sequential infusion rates of 0.5, 1.0, 1.5, and 2.0 mg/kg/h, with each level maintained for 5 minutes, until loss of responsiveness is achieved. EEG will be recorded during wakefulness, anesthetic induction, loss of responsiveness, and recovery. Pediatric surgical participants in this group will undergo EEG recording during anesthesia in the operating room.

Remimazolam will be administered intravenously in young and older healthy volunteers at sequential infusion rates of 0.5, 1.0, 1.5, and 2.0 mg/kg/h, with each level maintained for 5 minutes, until protocol-defined loss of responsiveness is achieved. Consciousness will be assessed every minute using the Modified Observer's Assessment of Alertness/Sedation (MOAA/S) scale, and EEG will be recorded throughout anesthetic induction and recovery.

Participants in this arm will receive esketamine. In young and older healthy volunteers, esketamine will be administered intravenously with the infusion rate increased by 0.10 mg/kg/h every 5 minutes, up to a maximum of 0.50 mg/kg/h, to induce loss of responsiveness. EEG will be recorded during wakefulness, anesthetic induction, loss of responsiveness, and recovery. Pediatric surgical participants in this group will undergo EEG recording during anesthesia in the operating room.

Esketamine will be administered intravenously in young and older healthy volunteers. The infusion rate will be increased by 0.10 mg/kg/h every 5 minutes, up to a maximum of 0.50 mg/kg/h, until protocol-defined loss of responsiveness is achieved. Consciousness will be assessed every minute using the Modified Observer's Assessment of Alertness/Sedation (MOAA/S) scale, and EEG will be recorded throughout anesthetic induction and recovery.

Participants in this arm will receive dexmedetomidine. In young and older healthy volunteers, dexmedetomidine will be administered intravenously with stepwise increases of 0.1 μg/kg/h, up to a maximum of 0.7 μg/kg/h, to induce loss of responsiveness. EEG will be recorded during wakefulness, anesthetic induction, loss of responsiveness, and recovery. Pediatric surgical participants in this group will undergo EEG recording during anesthesia in the operating room.

Dexmedetomidine will be administered intravenously in young and older healthy volunteers with stepwise increases in the infusion rate of 0.1 μg/kg/h, up to a maximum of 0.7 μg/kg/h, until protocol-defined loss of responsiveness is achieved. Consciousness will be assessed every minute using the Modified Observer's Assessment of Alertness/Sedation (MOAA/S) scale, and EEG will be recorded throughout anesthetic induction and recovery.

Continuous resting-state EEG will be quantified across awake baseline, anesthetic induction, loss of responsiveness, maintenance, emergence, and recovery. Prespecified measures include delta power (0.5-4 Hz), alpha power (8-12 Hz), theta/alpha power ratio, slow-wave index (SWI), burst suppression ratio (BSR), autocorrelation window at the first zero crossing (ACW-0), EEG complexity (gMLZC), and alpha-band (8-13 Hz) functional connectivity/network measures, including wPLI/dwPLI² and weighted global efficiency. Each measure will be analyzed separately.

Auditory-evoked EEG will be quantified across awake baseline, anesthetic induction, loss of responsiveness, maintenance, emergence, and recovery. Event-related potential (ERP) measures include N100, mismatch negativity (MMN), N400, and P300 amplitudes and their state-related temporal dynamics. Stimuli include pure tones, white noise, characters, words, sentences, another person's name, and the participant's own name. Time-frequency measures include event-related theta-band activity (4-8 Hz), alpha-band activity (8-12 Hz), and intertrial phase coherence. Source-space analyses will localize N400 and P300 generators, and single-trial ERP amplitudes will be analyzed in relation to behavioral responsiveness. Each measure will be analyzed separately.