CNN's Sanjay Gupta explains the phenomenon of déjà vu. <script type='text/javascript' src='http://z.cdn.turner.com/cnn/van/resources/scripts/van-widgets.js'></script> <div data-cnnvansinglewidget data-affiliate='gantdaily' data-videoid='health/2015/12/29/your-brain-on-deja-vu-sanjay-gupta-orig.cnn' data-size='auto' data-autostart='true'></div>