Sleep is often defined as a neurobehavioral state characterized by sensory and motor disconnection from the external environment. While awareness of the external environment fades during sleep, the brain continues to process stimuli from internal and external sources. On occasion, these stimuli are integrated into consciousness and reflected in dream content, indicating that sensory disconnection during sleep is not always perfect. Moreover, disconnected conscious experiences frequently occur across all sleep stages and throughout the night in the form of internally generated hallucinatory subjective experiences, broadly referred to as dreams.
Notably, non-rapid eye movement (NREM) sleep shares similar neural mechanisms and neurophysiological markers with unresponsiveness induced by anesthetics. Evidence from our recent experiments demonstrates that light experimental anesthesia is associated with a comparable recall rate and content of disconnected experiences to those observed in NREM sleep dreams. Interestingly, a frequently reported feature of these experiences was episodic memory incorporation of the research setting. In contrast, explicit recall of sensory stimuli presented during experimental anaesthesia was rare, suggestive of either sensory disconnection or amnesia.
Both experimental anesthesia and sleep provide valuable frameworks for investigating how the brain creates (disconnected) conscious experiences, and how sensory disconnectedness is induced on the neural level and reflected in subjective experiences. In this presentation, I will review a series of studies utilizing experimental anaesthesia and sleep to study consciousness in unresponsive states. I will argue that the boundary between sensory connectedness and disconnectedness is not absolute but rather exists on a continuum, encompassing a grey zone where internally and externally generated experiences become entangled with each other and with episodic memories. This nuanced interaction complicates the identification of the neural correlates of sensory disconnection.
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