 The study period will be as long as invasive neuromonitoring is clinically indicated following severe TBI. As outlined in the background section, previous investigations of CSD have ended after a 7-day recording period. The shorter recording period has likely hindered a full understanding of the natural history of CSDs following severe TBI and therefore a full understanding of the pathophysiological factors that produce these events. There will be minimal additional risk to extending the recording period to the full period of clinically indicated invasive neuromonitoring as subdural recording is routinely performed at the University of Minnesota for epilepsy monitoring for 4-6 weeks without a documented serious complication. 