Clinical studies We are exploring the role of damage to specific brain areas, including the reward pathways, in the genesis of fatigue, amotivation, and other common, unexplained complaints of military personnel returning from deployment. This study combines intensive neuroimaging with standard and experimental behavioral measures focusing on effort generation. Another study in collaboration with the Naval Medical Research Center and the Walter Reed Army Institute of Research is looking for evidence of behavioral and brain changes in military personnel with occupational exposure to blast. Neurophysiological probe studies We have successfully shown changes in the motor cortex response to transcranial magnetic stimulation (TMS) that are linked to the expectation of reward. We are currently studying how this response varies with expected reward probability. Studies are also underway using physiological responses to see how expected effort acts to discount anticipated reward. We are using magnetic resonance spectroscopy to explore how motor cortex gamma aminobutyric acid (GABA) concentration changes during learning and whether this correlates with neurophysiological phenotypes (responses to TMS) across individuals. This work is partially funded by the Center for Neuroscience and Regenerative Medicine, at the Uniformed Services University of the Health Sciences. We are using theta-burst transcranial magnetic stimulation of the motor and premotor cortex to produce a temporary impairment on learning tasks, as a model for traumatic brain injury. We then see whether adding various forms of reward to the same tasks can overcome this temporary blockade. These investigations will also be carried out with the addition of functional MRI, to look for the effect of theta burst stimulation and reward on patterns of activation. We have successfully completed a pilot study, funded by the Center for Neuroscience and Regenerative Medicine, at the Uniformed Services University of the Health Sciences, we are investigating the ability of near-infrared spectroscopy to detect activation of the frontal pole of the brain by a simple task, which has been previously validated using functional MRI. The goal is to provide a simple, computerized, portable, and robust system for detecting mild head injuries.

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