This study is designed to evaluate the high-frequency range deep brain oscillations (HFO) as pathologic markers in patients undergoing deep brain stimulation for epilepsy or Parkinson disease. Newly developed technology allows for the chronic recording of these brain signals at the same time as clinical stimulation is occuring. We will learn both whether these HFO correlate with disease activity and whether the HFO change in response to ongoing stimulation (potentially giving insight into the underlying mechanism of action of DBS).