When you repair electronics, you frequently test individual parts to see how they affect the whole. Why not try that with the brain? Stanford is doing just that. It developed a technique that fires specific kinds of neurons to map the brain and identify problems caused by Parkinson's and other diseases. The approach first uses optogenetics to make neurons activate in response to light, and follows up with a functional MRI scan to look for the increased blood flow that indicates activity in other brain regions. A computational analysis maps that particular neural circuit and helps determine its role.