Most Parkinson's disease (PD) patients eventually develop dementia, which is the most burdensome aspect of this progressively worsening condition. Mild cognitive impairments often indicate imminent dementia, but the 2 to 20-year time course poses a major problem for medical interventions, as brain changes associated with dementia in PD are still poorly understood. Recent evidence, however, suggests that neurodegenerative diseases such as PD progress along discrete brain networks. One important network, known as the 'default mode network', appears particularly susceptible to neurodegeneration. We will examine this network to determine if its disruption can specify which PD patients are vulnerable to progression to dementia within the next two years. A sophisticated but readily available brain imaging technique, called resting state functional imaging, will be used. These measures will assist in the selection of the most suitable patients for new treatments that may delay or prevent subsequent dementia in this vulnerable population.