Don't get me wrong. I’m not saying all visual phenomena seen with the eyes have a natural cause. I just think a lot of it has, and to mistake natural occurrences with something more is to be wandering up a blind alley.
People seem more then happy to disregard the ability of the mind to drink order from seeming randomness, and to find patterns where there really is none. For example, many people notice the effect of blood cells travelling through the myriad vessels of the retina. These show as sparkling dots, some look bright, or even coloured.
Arguments that these are more than blood vessels usually come across as they move in specific ways, they don't move randomly, or even they change colours.
Firstly, there are probably several million of these cells in the visual field at any one time, but we only notice a fraction, perhaps based upon their direction, or even a pattern the mind creates, and so notices cells which meet that criteria are seen.
This would give the perception of these cells/lights moving in certain directions, perhaps even being directed by us, as our intent to change their direction would make us notices those which match this criteria.
Secondly, stare at anything for long enough, and the depletion of rods and cells starts doing strange things. We see colours, things imprint more immediately, and we even see moving coloured lights.
For example, taking LSD will generally bring on a ‘pixilation’ of the visual field and produce moving lights. It’s as though even looking at something for an instant produces the effect of staring at something unwaveringly for 5 minutes when baseline.
If one starts noticing these phenomena, it takes them less to see them in the future. It seems actively seeking out certain stimulus changes our perceptual filters over time.
So these moving lights etc could possibly be more pronounced and contain coloured elements over time. (This has happened to me – and the opposite when I actively sought to ignore these things, and their presence reduced over months).
They might appear sentient because the thing noticing the patterns is sentient (us) .

I have read that people discount cone/rod depletion as the reason for seeing after images, due to them being able to see them instantly now (I see them instantly), whereas it took up to 20 seconds before. I think this is once again the changing of filters over time, forcing the brain to notice things usually ignored by conscious awareness, until we can see such things instantly.

Edit: Floaters are dead cells in the eyes, the lights are bloodcells traveling across the retina - they can look like a wind of particles flowing around.

Edit: Regarding the sparkly mist (cone/rod discharge) becoming more pronounced in trance could be explained by the nature of trance. Trance by definition is the reduction of external stimuli, which magnifies the internal processes. So as other sensory perceptions diminish, the mind latches on to those which remain (the sparkly lights). And as we near sleep states, hypongogic processes can be added to this effect to create some interesting results.