Listening to Variations VII today, there was a moment when I couldn't figure out exactly what was going on. I was hearing static and a pulsing sound--or static OR pulsing. I didn't know if I was hearing both or only one. One second I could hear one and not the other, and the next the other was the dominant noise. This could have been part of the piece, but I really thought my mind was playing tricks on me. I'm going to assume that the piece was designed that way--so that, if you concentrated on one sound, you would hear one 'melody', as far as a melody can get in such a piece. If you shift your focus to one of the many other sounds, you hear something completely different.
The only way I can really think to explain this the way I'm thinking is visual. There is a moving optical illusion I saw a while back. A shadow of a girl has one foot pointed out and is slowly revolving in a circle. Depending on the mindset of the person watching it, and how that person views the picture, the girl could be rotating clockwise or counterclockwise. If you find just the right way to shift your focus, she suddenly stops and begins turning the other way. I started thinking of this when listening to Variations VII.
There were so many sounds at one time, and just to 'cope' with it I automatically picked out one sound to concentrate on. Listening to that sound--static, or a fan--makes you start hearing a pattern to a sound you would normally try and block out. Taking in one sound made all the others fall into a pattern behind it. Suddenly switching to concentrate on another sound produces something completely different and the piece takes on something completely apart from what it had sounded like before. I don't know if it was just me, but I think if thirty different people heard this piece, they would all pick out thirty different noises that seemed to be the predominant sound in the piece for them.