It is now widely accepted that cognitive functions depend on the integrated
operation of large-scale distributed brain networks. Recent methodological
advances allow both structural and functional connectivity within these
networks to be studied non-invasively in vivo. These approaches hold the
promise of dramatically extending our understanding of the impact of
traumatic brain injury (TBI) on cognition, which should help determine
strategic targets for the rehabilitation of individuals with TBI. In this thesis, I
present three studies that combine structural and functional magnetic
resonance imaging to test the general hypothesis that cognitive deficits after
TBI arise from structural disconnection within brain networks that mediate
cognitive functions. In the first study, I demonstrate that sustained attention
deficits in TBI patients are related to a failure to regulate activity within a
‘default-mode’ network (DMN) thought to be involved, among others, in
internally directed processes such as self-referential thought. In addition,
these deficits can be predicted by the functional and structural connectivity
within the DMN. Next, I present a study investigating the neural basis for
inhibitory control in healthy subjects using a modified version of the Stop
Signal Task (SST). This study allows a clear distinction between attentional
and response inhibition processes, and paves the way for my last study,
which investigates inhibitory deficits after TBI. In this study, I demonstrate that
a failure of DMN deactivation during response inhibition is associated with
impaired inhibitory performance in TBI patients. The ability to efficiently
regulate the DMN can be predicted by the structural integrity within a remote
brain network previously proposed to be involved in switching between internally and externally directed attention. This work identifies DMN
dysfunction as underlying various cognitive deficits after TBI, and confirms the
relevance of white matter damage in the development of brain dysfunctions
after TBI.