The investigators suspect that mathematical analysis of the resting heart rate may provide some insight into brain metabolism. The brain controls heart rate in response to changes in blood pressure and blood gases like carbon dioxide and oxygen. Tight control of heart rate is necessary to make sure that the brain has the right mix of fuel and air. Because the brain can't respond instantly to changes in its fuel supply, this system acting as a biological carburetor has a natural oscillatory rhythm that can be monitored just like frequencies on the radio.