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a steady decrease in striatal activation across repeated blocks of a processing speed task in healthy participants. This is consistent with evidence from other neuroimaging studies that show healthy adults relying on striatal mechanisms early during task performance. In contrast, both MS and TBI participants showed increased or stable striatal activity across the task blocks, with a significant increase in PFC activation. This pattern of brain activity was observed despite no group differences in performance accuracy between healthy participants and MS/TBI participants. Further, this increase in striatal activation correlated with subjective CF 13 . Taken together, these data suggest that individuals with CF might have to recruit greater neural resources in order to maintain performance comparable to healthy individuals. Engagement of additional neural resources might result in increased effort leading one to feel CF. Indeed, it has been suggested that CF might be due to an effort-reward imbalance, i.e. an imbalance between the perceived amount of effort required by an action and the amount of reward that results from the action. That is, CF might result from inappropriate effort output and the perception that the outcome is not worth the effort 14 . In support of this hypothesis, neurophysiological studies with laboratory animals show that striatal lesions remove animals' preference to work for a larger food reward 15 . Such goal-directed behavior (exerting effort to obtain a reward) has been shown to depend on the neurotransmitter dopamine, suggesting a link between fatigue and dopamine levels in the brain. Several clinical trials showed the effectiveness of dopamine medication in reducing CF in various clinical populations, such as TBI, Parkinson's disease and chronic fatigue syndrome. The influence of dopamine on fatigue and associated brain activity in individuals with MS has not been investigated, either from pharmacological or from non-pharmacological perspectives. Our lab is currently conducting a clinical trial to see the effectiveness of dopaminergic medication on CF in MS. In addition, we are also investigating whether CF can be reduced through a non- pharmacological method (i.e. by means of reward presentation, a manipulation that has been shown to result in striatal dopamine release and changes in striatal activation). Preliminary evidence indeed shows that in both MS and healthy control participants, fatigue decreases when participants have an opportunity to obtain a reward 16 . Take Home Practice Points: 1. Subjective cognitive fatigue does not correlate with objective performance. 2. Time on task may affect subjective fatigue but not objective performance. 3. Subjective "state" cognitive fatigue does not correlate with traditional neuropsychological measures of "trait" fatigue. 4. Reward may reduce subjective cognitive fatigue, but additional research is necessary. 5. Dopamine imbalance might cause fatigue in clinical populations; more cross-discipline research is needed. 26 | Bulletin vol. 29 no. 1