Mild head injury or concussion accounts for as many as 90% of all traumatic brain injuries and can sometime result in long-lasting and disabling post-concussion symptoms (PCS), even in the absence of detectable structural damage to the brain by morphological imaging. To date, the nature of persistent PCS following mild head injury remains poorly understood as objective and scientifically valid diagnostic tools are practically absent. This, in turn, likely accounts for missed diagnoses, prevents accurate assessment of the severity of the injury and creates difficulties for achieving proper patient management. / This dissertation describes 4 functional magnetic resonance imaging (fMRI) studies designed to establish whether there is a pathological basis to persistent PCS following cerebral concussion in male athletes. Study 1 was intended to evaluate the feasibility of using fMRI to detect changes in brain activation following concussion. The results showed that concussed athletes displayed reduced prefrontal activities compared to a healthy control group when performing a working memory task. In Study 2, we examined the relationship between self-reported PCS and the pattern of brain activity measured by fMRI. The severity of self-reported PCS was found to be associated with blood oxygen level dependent (BOLD) activation patterns in the region of interest: the higher the PCS score the lower the BOLD signal. In Study 3, we used serial fMRI approach to investigate brain activation pattern in the course of injury recovery. The results showed that symptom resolution was accompanied by a return to normal BOLD response patterns. Study 4 was designed to investigate the nature of depression, a common PCS, following concussion. Concussed athletes with depression showed less activation in the dorsolateral prefrontal cortex, and attenuated deactivation in the medial prefrontal region, consistent with functional neuroimaging findings in major depression. Again, depression severity correlated with the strength of the fMRI signal in the region of interests. / Taken together, these results demonstrate the value of functional MRI in the evaluation of cerebral concussion, and provide evidence of an underlying pathology in persistent PCS following concussion.

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