Behavioral and neural correlates of chronic blast-related mild traumatic brain injury
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Blast-related mild traumatic brain injury (mTBI) is a common injury among Operation Enduring Freedom/Operation Iraqi Freedom (OEF/OIF) veterans due to the frequent use of improvised explosive devices (IEDs). A significant minority of veterans with blast-related mTBI complain of postconcussion symptoms (PCS) and cognitive difficulties, even years after the injury. Studies have suggested that these behavioral sequelae are primarily linked to mental health disorders such as posttraumatic stress disorder (PTSD). However, mTBI is associated with neural changes and the impact of these changes on behavioral sequelae is unclear. As such, this dissertation had three goals. First, this dissertation assessed whether the severity of PCS in blast-exposed individuals is associated with the extent of mTBI-related neural injury. Results revealed that individuals with mTBI with loss of consciousness (LOC) had significantly more white matter abnormalities than no-TBI controls and that these white matter abnormalities were spatially variable across individuals. Importantly, the extent of white matter abnormality was associated with physical PCS severity and mediated the relationship between mTBI with LOC and physical PCS. Second, this dissertation examined whether these white matter abnormalities were also associated with overall cognitive impairment. In light of the observed variability in white matter injury, a measure of overall cognitive status that takes into account heterogeneity of cognitive impairment was used. Results showed that the extent of white matter abnormality was associated with cognitive status and mediated the relationship between mTBI with LOC and cognitive impairment. Third, this dissertation examined performance and brain function in the context of an experimental measure of cognitive control known to be sensitive to residual effects of mTBI. Results revealed that although behavioral performance was similar across groups, the mTBI group had enhanced functional connectivity between brain networks important for task performance, suggesting a potential compensatory mechanism in mTBI. Together, the findings of this dissertation suggest that mTBI is associated with structural and functional connectivity alterations years after the injury. Further, this dissertation suggests that whereas structural connectivity changes may have negative behavioral consequences, changes in functional connectivity may serve as a compensatory mechanism for successful performance.