This copy is for your personal, non-commercial use only. To order presentation-ready copies for distribution to your colleagues, clients or customers visit http://www.djreprints.com. Advanced imaging technology can pinpoint regions of the brain that light up when stimulated by fear and euphoria. But you don’t need a brain scan to see the market toying with those emotions: The Dow Jones Industrial Average plunged about 5,000 points from early October to Christmas last year, only to recover nearly 3,000 points in trading through Jan. 30, one of the biggest whipsaws in recent history. Signals like this can be gut wrenching.