Not only can brain activation be controlled though consciousness (which is expected under most reductionist accounts of the mind-brain problem) and measured in the lab (as @Jeff's answer showed) but it can actually be used as an interface!
Erik Ramsey is locked-in syndrome patient and is incapable of movement apart from his eyes. However, he has control of his brain enough to be distinguished by electrodes implanted into his cerebral cortex (Guenther, 2009). Although the sensors were implanted in the motor-area responsible for speech (which you would expect typical inviduals to have control over, since they can speak), the understanding of how to decode this is not full. Thus, the patient had to train in order to adapt to the system:
Accuracy of the volunteer's vowel productions with the synthesizer improved quickly with practice, with a 25% improvement in average hit rate (from 45% to 70%) and 46% decrease in average endpoint error from the first to the last block of a three-vowel task.
In a much more dramatic experiment, Adrian Owen has used brain imaging techniques as means of communicating with vegetative-state patients. It is known that visualizing playing-tennis and walking around the house produce very distinct (to an fMRI) activation in the brain. So owed used that capability to allow a vegetative-state patient to answer yes-or-no questions. He asked the patient to imagine playing tennis for yes, navigating the house for no (Monti et al., 2010).
Of course, this is not an answer to the exact question you asked, but by thinking about these activities the patient is effective the blood flow in his brain significantly enough to be measured by fMRI and used as a form of communication. Of course, some do raise concerns over the philosophical questions related to consciousness in these patients, to better understand consciousness in general, take a look at:
What are current neuronal explanations and models of 'consciousness'?
Guenther, F.H., Brumberg, J.S., Wright, E.J., Nieto-Castanon, A., Tourville, J.A., Panko, M., Law, R., Siebert, S.A., Bartels, J.L., Andreasen, D.S., Ehirim, P., Mao, H., & Kennedy, P.R. (2009). A wireless brain-machine interface for real-time speech synthesis. PLoS ONE, 4(12)
Monti MM, Vanhaudenhuyse A, Coleman MR, Boly M, Pickard JD, Tshibanda L, Owen AM, & Laureys S. (2010). Willful modulation of brain activity in disorders of consciousness. New England Journal of Medicine, 362(7):579-89.