A repeated awakening study exploring the capacity of complexity measures to capture dreaming during propofol sedation

People under general anesthesia are often assumed to be unconscious, but some experience dreams. In 20 healthy participants sedated with propofol, EEG was recorded at rest and with TMS perturbations. Participants were repeatedly awakened from deep sedation and asked if they had an experience just before waking. Of 52 attempted awakenings, 24 yielded reports of an experience, 5 reports of no experience, and 23 were unarousable or gave incoherent reports. Two EEG complexity measures—the state transitions perturbational complexity index and single-channel Lempel-Ziv complexity—decreased from awake to sedated states, but did not differ between periods with and without reported experiences within sedation. The authors discuss interpretations and limitations.

Study at a glance

Abstract

Abstract Patients undergoing general anesthesia are often assumed to be unconscious. However, it is known that conscious experiences in the form of dreams occur even in unresponsive states induced by anesthetics. Here, we recorded resting state electroencephalography (EEG) as well as EEG combined with TMS perturbations in 20 healthy participants during propofol sedation. Participants were repeatedly awoken from deep sedation and asked immediately whether they had experiences just before waking up and what they experienced. Out of the 52 attempted awakenings in this study, 24 produced reports of having had an experience, while there were 5 reports of no experience. In the remaining 23 attempts, the subject was either unarousable or the report too incoherent to provide information about their experience prior to awakening. We then tested whether two different consciousness measures based on EEG complexity—the state transitions perturbational complexity index (PCI st ) and single-channel Lempel-Ziv complexity (LZc)—differed between awakenings with and without experience. While our study confirms earlier findings that the EEG complexity measures significantly decrease from the awake state to the sedated state, we find no evidence that these measures differ between periods associated with reports of dreaming and non-dreaming, within the sedated state. A few interpretations and limitations are discussed.