Conscious information processing during and after recovery from deep anaesthesia
Citation:Taylor, Nicola, Conscious information processing during and after recovery from deep anaesthesia, Trinity College Dublin, School of Psychology, Institute of Neuroscience, 2021
Nicola_Taylor_11417538_MSc_Thesis_2021.pdf (PDF) 3.019Mb
This thesis aimed to answer two important questions in anaesthesia research. Firstly, despite an abundance of studies examining brain changes upon abolition of consciousness by anaesthesia, very few have focused on how these brain features reconstitute upon the recovery of consciousness. I addressed this question in Chapter 2. Secondly, there is very limited knowledge on brain mechanisms that might enable the preservation of conscious awareness, in rare cases, under general anaesthesia. I addressed this question in Chapter 3. Naturalistic paradigms provide an opportunity to examine the neural dynamics underlying changes in higher-order cognitive processes during and after anaesthesia. This thesis used a unique dataset where consciousness was abolished with deep propofol anaesthesia in a healthy group of 16 participants inside the 3T MRI scanner, while participants were exposed to a complex, plot-driven narrative (from the movie Taken) as well as during the resting state. Scans took place while participants were awake, under deep anaesthesia, and after they had recovered behavioural responsivity. In Chapter 2, I calculated functional connectivity (FC) within and between seven major networks across the brain. I found that during Taken, in contrast to the resting state, the FC increased significantly under deep anaesthesia and remained elevated during recovery relative to the awake state. Furthermore, widespread inter-subject correlation during the story, a measure of stimulus-driven information processing, did not restore back to awake levels in the recovery state. My results suggest that, immediately upon recovery, complex information processing is impaired relative to typical wakefulness, and both measurements of brain activity in the resting state and clinical behavioural scales fail to detect this effect. In Chapter 3, an independent group of participants (N=25) listened to the same story outside of the scanner and rated its suspense at two-second intervals. I first established that these suspense ratings were highly correlated across individuals and tracked plot developments that required ongoing higher-order cognitive processes, such as sustained attention and working memory, to follow. Therefore, I could use the average suspense rating as a psychological descriptor of narrative understanding over time. In wakeful participants, suspense ratings predicted robust activation, at the group and individual level, of a set of brain areas, including regions of the auditory attention and language processing network (the left/ right superior temporal gyrus/ sulcus, supramarginal gyrus) and the salience network (the left/right anterior insular gyrus, cingulate sulcus, brainstem areas, and the thalamus). At the group level, this activation was abolished during deep sedation. However, 1/16 participants (6%) during deep anaesthesia demonstrated the complete activity patterns associated with high-order auditory attention and cognitive and emotional appraisal of the story in wakeful participants, suggesting that this individual processed and understood the story similarly to awake participants. Traditional methods of detecting awareness under anaesthesia put its incidence at 0.1-0.2%, however they may underestimate its true incidence due to their reliance on patient recall. The isolated forearm technique (IFT) does not require explicit recall and produces higher estimates of up to 4.6%, however a positive result on the IFT requires several capacities in addition to conscious awareness, which may be impacted by anaesthetic agents. The higher rate reported in my study may reflect the lower task demands of understanding an auditory story compared to the command-following required for IFT. My findings provide a novel marker for measuring awareness under general anaesthesia, and underscore the importance of developing methods of assessing consciousness that do not require behavioural response or explicit recall. Future research should aim to develop methods that translate the current approach to cheaper, more portable technologies.
Irish Research Council
Author: Taylor, Nicola
Type of material:Thesis
Availability:Full text available