I would like to respond to this discussion too. In my experience, spinal breathing usually is a "felt" experience and has never been a visual experience. There is a pronounced experience of prana flowing from root up to the head on inhaling, and a flow of energy back down from head to root on exhalation. In fact, I have concluded that this flow occur continually with each breath, but we just don't usually notice it. Because, it is often present in awareness whether in formal meditation or just any old time. It is quite nice. It is associated with movement at the heart center which will throb, and crown activity also of a throbbing enjoyable sensation. It is enhanced by ancient mantra practices, and by pranayama.
Whether some people experience some vision of sushumna is beyond me to say, but definitely not my experience which is more a felt sensation. Just one yogi's experience to report, that's all.