In a couple of papers, the authors demonstrate that expected reward does modulate the activity in the primary motor cortex. Their technique was rather simple but intelligent. They simulate a slot machine on a computer screen in front of the subjects. If the first two symbols matched, there was a 50% chance that the third symbol would match too. By delivering a double pulse TMS in between the second and third symbols appearance, they were able to capture the influence of the reward function on the primary motor cortex activity. How this reward signal depends on dopamine will be the topic of their next paper.
Kapogiannis D, Campion P, Grafman J, Wassermann EM
Reward-related activity in the human motor cortex - European Journal of Neuroscience 27 1836-1842 2008