Dementia is the leading cause of nursing home placement in Parkinson's disease (PD) yet little is known about the cause(s) of cognitive dysfunction in PD and there are no effective treatments. The investigators preliminary data and other published studies suggest that abnormalities in brain activity involving networks important for normal thinking and memory may contribute to cognitive dysfunction in PD and may represent a target for treatment. This proposal will identify abnormalities in cortical activity related to cognitive dysfunction in PD using magnetoencephalography and will perform a randomized control trial of bifrontal repetitive transcranial magnetic stimulation to determine the therapeutic potential of modulating this brain activity.