Consciousness—subjective experience—persists during sleep and anesthesia, as evidenced by dreaming. A defining feature of dreaming is disconnection from the environment. Anesthesia aims to prevent the experience of surgery (connected consciousness) by inducing either unconsciousness or disconnection. The isolated forearm technique reveals that consciousness, connectedness, and responsiveness can uncouple during anesthesia; under clinical conditions, a median 37% of patients demonstrate connected consciousness. Potential neurobiological constructs explain this: during light anesthesia, subcortical mechanisms for spontaneous behavioral responsiveness are disabled, but information integration within the corticothalamic network continues producing consciousness, while unperturbed norepinephrinergic signaling maintains connectedness. These concepts emphasize the need for anesthetic regimens and depth-of-anesthesia monitors targeting mechanisms of consciousness, connectedness, and responsiveness.
Sleep and anesthesia alter conscious experience, which can be absent (unconsciousness) or take the form of dreaming where sensory stimuli are not incorporated (disconnected consciousness). Using transcranial magnetic stimulation-electroencephalography over parietal regions, evoked alpha power (8-12 Hz) decreased during disconnected consciousness in rapid eye movement sleep and ketamine anesthesia compared to wakefulness. In unconscious states of propofol anesthesia and non-rapid eye movement sleep, evoked low-gamma power (30-40 Hz) decreased compared to wakefulness or disconnected consciousness. These findings, confirmed with dream reports from serial awakenings, suggest suppression of evoked alpha activity may mark sensory disconnection.