Dopamine in Motor Cortex Is Necessary for Skill Learning and Synaptic Plasticity and published in PLOS one.
In this paper, the authors show that elimination of dopaminergic projections to M1 impairs skill learning. More specifically, the authors observed that rats with impaired dopaminergic projections to the primary motor cortex did not learn as well as control rats in a food pellet retrieval task. They showed that 1) this was specific to dopaminergic projections and that 2) skill acquisition was restored upon dopamine substitution (levodopa).
These results might explain why dopamine taken orally before physiotherapy might help recovery of motor skills in stroke patients.
In the discussion, the authors debate about the possible existence of projections between VTA and M1 (second-to-last paragraph of the discussion). Kapogiannis and colleagues recently published a paper entitle "reward-related activity in the human motor cortex" where they show, using transcranial magnetic stimulation, that rewards may influence excitability of the motor cortex. Because reward and dopamine has been linked in many studies, it suggests that the modulation of M1 excitability might arise from dopamine projections from VTA to M1 (they confirm the role of this projections in their next to-be-published study).
This study highlights that those projections facilitate learning, hence linking the motor cortex to the reinforcement learning or reward-based learning theory. These theories would suggest that you learn better when you're rewarded and that the food pellets were rewarding. An interesting question is what would happen if the food pellets were not as rewarding anymore (given that motivation is intact). I believe that if Luft and colleagues were able to observe an effect of reward magnitude on success rate in their task, elimination of the dopaminergic terminals would abolish this effect.