title: Hyperconnectivity is a fundamental response to neurological disruption.
abstract: OBJECTIVE
  In the cognitive and clinical neurosciences, the past decade has been marked by dramatic growth in a literature examining brain "connectivity" using noninvasive methods. We offer a critical review of the blood oxygen level dependent functional MRI (BOLD fMRI) literature examining neural connectivity changes in neurological disorders with focus on brain injury and dementia. The goal is to demonstrate that there are identifiable shifts in local and large-scale network connectivity that can be predicted by the degree of pathology. We anticipate that the most common network response to neurological insult is hyperconnectivity but that this response depends upon demand and resource availability.
  
  
  METHODS
  To examine this hypothesis, we initially reviewed the results from 1,426 studies examining functional brain connectivity in individuals diagnosed with multiple sclerosis, traumatic brain injury, mild cognitive impairment, and Alzheimer's disease. Based upon inclusionary criteria, 126 studies were included for detailed analysis.
  
  
  RESULTS
  RESULTS from 126 studies examining local and whole brain connectivity demonstrated increased connectivity in traumatic brain injury and multiple sclerosis. This finding is juxtaposed with findings in mild cognitive impairment and Alzheimer's disease where there is a shift to diminished connectivity as degeneration progresses.
  
  
  CONCLUSIONS
  This summary of the functional imaging literature using fMRI methods reveals that hyperconnectivity is a common response to neurological disruption and that it may be differentially observable across brain regions. We discuss the factors contributing to both hyper- and hypoconnectivity results after neurological disruption and the implications these findings have for network plasticity.
authors.affiliation: Department of Psychology, Pennsylvania State University.
authors.forename: Frank G
authors.initials: FG
authors.lastname: Hillary
authors.affiliation: Department of Psychology, Pennsylvania State University.
authors.forename: Cristina A
authors.initials: CA
authors.lastname: Roman
authors.affiliation: Department of Psychology, Pennsylvania State University.
authors.forename: Umesh
authors.initials: U
authors.lastname: Venkatesan
authors.affiliation: Department of Mathematics, Pennsylvania State University.
authors.forename: Sarah M
authors.initials: SM
authors.lastname: Rajtmajer
authors.affiliation: Department of Physics, Complutense University of Madrid.
authors.forename: Ricardo
authors.initials: R
authors.lastname: Bajo
authors.affiliation: Department of Physics, Complutense University of Madrid.
authors.forename: Nazareth D
authors.initials: ND
authors.lastname: Castellanos
mesh_terms: D000368:Aged; D000544:Alzheimer Disease / Q000503:physiopathology; D001921:Brain / Q000503:physiopathology*; D001930:Brain Injuries / Q000503:physiopathology; D002493:Central Nervous System Diseases / Q000503:physiopathology*; D060825:Cognitive Dysfunction / Q000503:physiopathology; D005260:Female; D006801:Humans; D008279:Magnetic Resonance Imaging / Q000379:methods; D008297:Male; D009103:Multiple Sclerosis / Q000503:physiopathology; D009434:Neural Pathways / Q000503:physiopathology*; D009473:Neuronal Plasticity; D010100:Oxygen / Q000097:blood
chemical_list: D010100:Oxygen