Discussion
hallucinatory, amnesic
We compared recall and content of subjective experiences reported after awakenings from anaesthetic-induced unresponsiveness and NREM sleep in the same male participants. As hypothesised, we found no significant differences in the recall rate or main content of experiences between anaesthesia and natural sleep, although disconnected experiences during anaesthesia tended to be more often dynamic than experiences during sleep. Our results are, in general, in line with previous findings regarding experiences in experimental anaesthesia and NREM sleep.Rousability did not differ between dexmedetomidine and propofol, but lower drug plasma concentrations were related to rousability with both drugs, as expected. Experiences with specific remembered content were reported in about two-thirds of the awakenings, and the reported experiences almost always reflected disconnected contents of consciousness. The recall of experiences after unresponsive periods (no recall, white report, or content report) or the type of recalled content (dreaming or memory incorporation) did not differ between anaesthetic agents or sleep stages N1, N2, and N3, but in the propofol group those who reported dreaming after waking up from the first unresponsive period had higher measured drug plasma concentrations than those who did not report dreaming. Memory incorporation, that is, contents that reflected some remembered aspects of the research environment but not direct awareness of it, was present in more than 80% of the reports in all awakenings (except for the N1 sleep stage). Interestingly, memory incorporation experiences were not related to drug plasma concentrations with either dexmedetomidine or propofol, even though a higher probability of sensory intrusions from the research environment could have been expected to be related to lower concentrations.Notably, in 50.9% of anaesthesia reports and 29.8% of sleep reports, dream-like content was present in the same report as memory incorporation content, that is, content related and unrelated to the research setting co-existed in the same report. Similarly, all six reports that included awareness aslo included memory incorporation and all but one dreaming. These findings not only add evidence that disconnected conscious experiences persist and are prevalent in unresponsive states, but they show that when probed with a detailed interview, experiences reflecting different degrees of connectedness can interweave. Experiences during both anaesthesia and sleep often superimposed Given that we cannot exclude the possibility that connected phenomenal consciousness was present in some of those occasions when memory incorporation was reported, our findings might bear some relevance for clinical practices. Administration of anaesthetics in the current study corresponds to mild-to-moderate sedation rather than surgical anaesthesia and both responsive and unresponsive sedation are widely used in clinical contexts. When unresponsive sedation is used with the hope of abolishing connectedness, it might be important to address retrospective recall to assess whether this goal was successfully achieved. In previous studies on dreaming during surgical anaesthesia, the contents of dreams have mostly been related to everyday life or the surgical setting and operating room.Although the amnesic effects of anaesthetics are well documented,The strengths of the present study lie in the direct comparison of experiences in two different unresponsive states utilising the same methods in a within-subject design, thus mitigating the effect of individual trait differences (such as retrospective memory for subjective experiences) on outcomes.The current study also suffers from several limitations. First, sample size was limited. However, the reported 95% CIs exclude large differences between the drug groups or between anaesthesia and sleep and suggest that our sample size was adequate to test our hypotheses. Second, our sample included only males because of the potential risks of exposing fertile females to radioactive tracers. We are unable to exclude potential sex differences on the outcomes, and future investigations on this topic are needed.As to other weaknesses in design, in Experiment 1 the full interviews were slightly delayed because of intervening PET scans. Delays might have accentuated the amnesic effects of anaesthetics and distorted the reports. To reduce memory bias, we presented the brief dream question immediately after arousal and conducted the full interview on average 7.5 min later. This, however, led to discrepancies in participants' answers. On a few occasions, participants answered ‘yes’ to the brief dream question (i.e. they recalled a dream) but in the full interview no longer recalled any content, which likely reflects the amnesic effects of the anaesthetics. On other occasions, participants were ‘unsure’ in the brief dream question but then reported dreaming in the full interview. Possibly, when participants had time to think about the experience during the PET scan, they recalled details that could not be immediately reported, that is, an immediate white report transformed into a content report. Another explanation might be that despite responsiveness tests that verified responsiveness before and after the PET scan, participants' state of consciousness fluctuated, and between the brief dream question and the full interview their contents of consciousness alternated between disconnected and connected experiences. Participants might have had brief episodes of internally generated thoughts and imagery during this responsive period, similar to mind wandering or sleep onset hypnagogia, and thus in the full interview they reported hallucinatory dream-like experiences that they had during the scan just before the full interview.The largest discrepancy was observed between answering ‘no’ to the brief dream question and reporting memory incorporation in the full interview. This discordance could point to the possibility that some memory incorporations occurred during the responsive period, between the brief dream question and full interview. Yet, memory incorporation experiences were not related to drug plasma concentrations, that is, lower concentrations did not lead to higher levels of connectedness and incorporation from the responsive period. Another possibility is that directly inquiring about the incorporation of the research environment in the interview might have led to over-representation of such content. However, memory incorporation rate in the present study is not substantially higher than in our previous study utilising immediate interviews,For future studies, we stress the importance of clarifying concepts to participants, asking unambiguous questions, and conducting immediate interviews to diminish the time lag between the experience and the report, and to mitigate amnesic effects, memory bias, and filling in the gaps by confabulation. We support use of thorough and transparently reported interview and classification procedures and clearly defined categories in research of anaesthesia-related experiences.