Deep brain stimulation (DBS) is an established treatment for advanced Parkinson's disease, medically refractory tremor, dystonia and obsessive compulsive disorder. Several hypotheses driven DBS trials are underway to study modulation of circuit dysfunction in other neurological and psychiatric disorders like epilepsy, Alzheimer's disease and depression. Recent reports suggest profound effects of DBS on the anatomy and function of downstream areas in the brain. For example electrical stimulation of limbic circuits is associated with increase in hippocampal neurogenesis. Similarly, stimulation of subthalamic nucleus (STN) or globus pallidus (GPi) results in activation of cortical motor circuits. Non-invasive imaging modalities are increasingly being employed in these investigations to better understand the effects of DBS on the structure and function of the brain. There have been important advances in MRI and we now have MRI which provides higher resolution and higher quality brain images. More specifically, the investigators propose to use MRI to perform functional magnetic resonance imaging (i.e. fMRI) to assess the effects of deep brain stimulation on brain function and to assess whether fMRI can be used as an adjunct to improve clinical practice in these patients.