Motor Cortex Reward Signaling in Parkinson Disease

The role of mesencephalic dopamine neurons in reward processing has been established in primates using electrophysiological techniques and in humans using functional neuroimaging. Their role is thought to be dual: i) they show sustained activity with the expectation of a future reward and ii) a phasic response after reward. Animal data indicate that these neurons, located in the midbrain areas A8-10, behave as a single functional unit when activated. They have rich projections to both the prefrontal and motor cortices where they synapse on interneurons and cortical pyramidal cells, producing primarily inhibition. Though their function is not fully understood, these projections clearly play an important role in motivation and learning. We have recently developed a pair of paradigms to detect a reward related signal in the primary motor cortex, where transcranial magnetic stimulation can be used to measure brief events. So far, we have tested healthy volunteers; we now intend to study Parkinson disease (PD) patients in order to help clarify the role of dopamine in effect of reward. The Parkinson patients will be tested both on and off levodopa/carbidopa and dopamine agonist medications. Our hypothesis is that the dopamine reward related signal will alter level of evocable inhibition in primary motor cortex. Using a behavioral paradigm that deliver intermittent reward, we aim to demonstrate, in treated and untreated PD patients a difference in the amount of cortical inhibition, i) when reward is expected compared to when reward is not expected and ii) after rewarded compared to unrewarded trials. We also aim to demonstrate a difference between their response and that of healthy controls.