The vision therapist had me hold a pencil out in front of me and slowly bring it towards my nose. I brought it towards my right eye instead. She pointed this out to me and had me do it again. I concentrated hard to bring it to my nose, but I brought the pencil towards my left eye this time. I don't know where the center of my face is! So now I've noticed anytime I think my face is centered with something (and I'm looking directly at it), my head is really turned slightly to the left so that I'm looking directly at it only with my right eye.
This is my sister wearing a pair of prism glasses from Vision Therapy.