Hey Mart,
It's good to see that I'm not the only one with these problems (or gifts, curses, whatever...). I know EXACTLY what you are talking about with what you are seeing, and no, they are not floaters. I can distincly tell the difference between floaters and these phosphmenes (psych terminology - documented), and these are definately not floaters. Floaters are non-voluntary and I ALWAYS see them, whereas the specks I have to WANT to perceive them. As for the brain lacking oxygen, when I focus my mind to see these specks, I have deep controlled breathing for relaxation, so lacking oxygen is certainly not causing this. I was also able to get my roomate to see them as well, you really need a mind of nothingless (think of a blank slate) and then focus on trying to percieve these things. Another note, I don't do drugs (aside from alcohol) nor have any medications, I party on weekends with friends, work hard at my studies during the week, overall, an average guy.
As for the black lines between fingers, I don't see that at all. Instead I see something that I can only explain as light 'bending' around my finger (like heat will bend light on the surface of the road on a hot day). As my finger moves across a backdrop, the images near the finger will get sucked closer to it as I move them around, much like how the sun's gravity well will change the path of light from a distant star to make the star seem like it has changed position when the sun's gravity has only altered it's course (einstein's relativity, smart bast'd).
Also, I found a similar anomaly with sound, preferrably white noise (or the sound of a fan or moving air...) when going to sleep. I found the sound would start to change from what I was hearing to something else. Some cases it would be parts of a conversation, ambient sounds you would hear on the street, pretty much anything. The only ticker was that when I had become aware the sound was no longer the sound of the static 'white noise', I would lose focus on it and I would revert hearing white noise again. To try this, simply put on some white noise or pink noise with some headphones and hit the hay. Try to clear your mind and wait... when your mind seems to drift away from this plane, you may notice strange things happening all around you.
However, I cannot perceive any of these strange occurrences randomly like I can with my floaters, I can pick those out anywhere, anytime. It's only when I focus my mind that I can see them. I think it's more of a state of consciousness than a physical problem, after all, look at what Einstein or Tesla came up with when they were able to focus the mind.
It may not be a scientific answer, but rather one of those theories that can remain a theory because there's no way of disproving it, or even proving it for that matter. Science does tell us that we do use some small percentage of our brains (~5%?), maybe concentration and focus is like working the brain muscle, allowing us to greater sensitivity or feeling to the world around us. After all, we know that there's a greater spectrum beyond visible light (ROYGBV) that we can't see, but some insects do.