216 Chapter 7 difficulty > moderate difficulty contrast in patients, but the corresponding behavioural abnormalities suggest future studies with increased power to demonstrate a significant neural effect. Larger participant groups are probably needed to be able to translate more behavioural results in differences in neural activation. No previous vigilance-related MRI studies in people with narcolepsy have been reported, making direct comparisons of our results impossible. Time-on-task effect Participants made significantly more mistakes in repetition 1 vs. repetition 4 in the moderate difficulty level. Interestingly, no between-group differences were found in the corresponding fMRI contrast. This could be related to the behavioural markers being more sensitive to short interruptions of instructions that were displayed between conditions, allowing the participants to recuperate. However, both time-on-task behavioural and fMRI differences were found comparing the two block halves of the higher difficulty level between groups. Controls, but not people with narcolepsy type 1, were able to activate further their task-positive networks when transitioning from the early half into the late half. Interestingly this is only found when vigilance demand is high. We suggest that maximal vigilance capacities have already been reached by people with narcolepsy type 1 in the early stage of the higher difficulty level, whereas controls are able to upregulate their cognitive effort to remain vigilant throughout the second half. It seems that people with narcolepsy type 1 experience problems transitioning from attention initiation to stable levels of attention. Interestingly, a similar stabilizing role has been proposed for the cingulo-opercular network during sleep, where its activity is positively correlated with stabilizing and deepening sleep through synchronization of the cyclic alternating pattern [300-302]. The question arises whether instability of the cingulo-opercular network as observed in our study during wake, could also play a role in disturbed sleep, potentially resulting in typical narcolepsy type 1 characteristics such as sleep state instability and fragmentation of nocturnal sleep. No studies have been published performing SART time-on-task analyses in people with narcolepsy, but previous clinical studies have reported similar cognitive difficulties in relation to lower vigilance capacities in people with narcolepsy [92]. The inability to sustain attention within blocks with high vigilance demand could resemble difficulties in daily life (e.g., studying, driving, working) experienced by people with narcolepsy type 1 [303].
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