Voidlike Black

I just finished reading the first part of Franny and Zooey (the Franny part) and there was one sentence that really struck me.

Then she placed her hands, vertically, over her eyes and pressed the heels hard, as though to paralyze the optic nerve and drown all images into a voidlike black.

I used to do this when I was little. If I couldn't fall asleep at night, I would lay on my stomach, lean on my elbows and press the the heels of my palms flat on my eyes. Everything would go black and against that voidlike black backdrop I would begin to see little shapes move to and fro, expand and contract, almost like a kaleidoscope. Sometimes I would try to control what I saw. If I wanted to see a pink blob, I'd see a pink blob. If I wanted it to move left, it would move left. I always wondered if other people did this. Now I know.