In Surely You're Joking, Mr. Feynman!, Richard Feynman recounts a similar "drifting" of the location of the ego while in a sensory deprivation tank:
"I tried and after a while I got my ego to go down through my neck into the middle of my chest. When a drop of water came down and hit me on the shoulder, I felt it "up there," above where "I" was. Every time a drop came I was startled a little bit, and my ego would jump back up through the neck to the usual place."