title: Graph Theoretical Analysis of BOLD Functional Connectivity during Human Sleep without EEG Monitoring.
abstract: BACKGROUND
  Functional brain networks of human have been revealed to have small-world properties by both analyzing electroencephalogram (EEG) and functional magnetic resonance imaging (fMRI) time series.
  
  
  RESULTS
  In our study, by using graph theoretical analysis, we attempted to investigate the changes of paralimbic-limbic cortex between wake and sleep states. Ten healthy young people were recruited to our experiment. Data from 2 subjects were excluded for the reason that they had not fallen asleep during the experiment. For each subject, blood oxygen level dependency (BOLD) images were acquired to analyze brain network, and peripheral pulse signals were obtained continuously to identify if the subject was in sleep periods. Results of fMRI showed that brain networks exhibited stronger small-world characteristics during sleep state as compared to wake state, which was in consistent with previous studies using EEG synchronization. Moreover, we observed that compared with wake state, paralimbic-limbic cortex had less connectivity with neocortical system and centrencephalic structure in sleep.
  
  
  CONCLUSIONS
  In conclusion, this is the first study, to our knowledge, has observed that small-world properties of brain functional networks altered when human sleeps without EEG synchronization. Moreover, we speculate that paralimbic-limbic cortex organization owns an efficient defense mechanism responsible for suppressing the external environment interference when humans sleep, which is consistent with the hypothesis that the paralimbic-limbic cortex may be functionally disconnected from brain regions which directly mediate their interactions with the external environment. Our findings also provide a reasonable explanation why stable sleep exhibits homeostasis which is far less susceptible to outside world.
authors.affiliation: Academy of Advanced Interdisciplinary Studies, Peking University, Beijing, China.
authors.forename: Jun
authors.initials: J
authors.lastname: Lv
authors.affiliation: School of Biomedical Engineering, Capital Medical University, Beijing, China.
authors.forename: Dongdong
authors.initials: D
authors.lastname: Liu
authors.affiliation: Dept. of Pulmonary Medicine, Peking University First Hospital, Beijing, China.
authors.forename: Jing
authors.initials: J
authors.lastname: Ma
authors.affiliation: Academy of Advanced Interdisciplinary Studies, Peking University, Beijing, China; Dept. of Radiology, Peking University First Hospital, Beijing, China.
authors.forename: Xiaoying
authors.initials: X
authors.lastname: Wang
authors.affiliation: Academy of Advanced Interdisciplinary Studies, Peking University, Beijing, China; College of Engineering, Peking University, Beijing, China.
authors.forename: Jue
authors.initials: J
authors.lastname: Zhang
mesh_terms: D000328:Adult; D001921:Brain / Q000502:physiology*; D001931:Brain Mapping; D004569:Electroencephalography; D005260:Female; D006801:Humans; D008279:Magnetic Resonance Imaging; D008297:Male; D009434:Neural Pathways; D010101:Oxygen Consumption; D012890:Sleep / Q000502:physiology*; D055815:Young Adult