345 Discussion limitations. Implementation of more specific MRI sequences such as magnetic resonance spectroscopy (MRS) and multishell DWI can also provide novel perspectives on the pathophysiologies of central disorders of hypersomnolence. In close collaboration with Concordia University (Montreal, Canada), we have recently launched two initiatives to overcome these limitations: a largescale harmonized cross-disorder neuroimaging collaboration between our two sites and the global Neuroimaging Consortium on Central Disorders of Hypersomnolence (NICHY) [104, 443]. Results from these projects are still unpublished and have therefore not been included as individual chapters in this dissertation. The setup and data acquisition of these initiatives were important components of the PhD trajectory underlying this dissertation and will hereafter be described. We have recently finalized data acquisition of a harmonized multicentre study including a drug-free age- and sex-matched sample of 30 individuals with narcolepsy type 1, 22 with narcolepsy type 2 and 35 with idiopathic hypersomnia, and 30 healthy controls which have been measured wellrested and after partial acute sleep restriction. Subjects have first undergone concurrent EEG-fMRI measurements during resting state, a prolonged version of the SART on the higher difficulty level, a face-name memory task with direct and delayed recall, a 30-minute napping period and the SART directly following waking up. A second MRI session included structural T1- and T2weighted imaging, multishell DWI and MRS tailored to quantify GABA in the medial prefrontal and posterior cingulate cortices as important DMN hubs. All subjects also completed clinical and cognitive assessments. Measurements were preceded by one-week actigraphy and sleep diary recordings, and for subjects included in Montreal, a repeated polysomnography and MSLT. This extensive dataset with detailed clinical information will allow for identifying disease-specific brain profiles through cross-disorder comparisons. Inclusion of sleep-deprived healthy individuals also provides the opportunity to assess whether findings are specific for central disorders of hypersomnolence or resulting from daytime sleepiness. Data acquisition has recently been finished and we are currently in the phase of data analysis. To confront the challenge of small sample sizes and inconsistent outcomes, we also established the neuroimaging consortium on central disorders of hypersomnolence (NICHY) [104, 443]. This initiative involves the global integration of existing and future MRI datasets related to narcolepsy and idiopathic hypersomnia to conduct cross-disorder comparisons among hypersomnolence diagnoses with unprecedented statistical power. Currently we have over 28 involved sites from 21 countries and five continents who 11
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