I first read about this back in the 1980s, in an issue of Science Digest. Couldn't find a link or reference on short notice, but here's something from the American Academy of Ophthalmology that explains the phenomenon, with an experiment to see the blood vessels in your eye:
Apparently, the brain tends to ignore visual stimuli that don't change over a short period of time, which allows you see "around" the blood vessels passing through the middle of your eye. By closing your eye, and moving a penlight around against your eyelid, you can make the vessels cast a shifting shadow on your retina that makes them visible.
The reason you usually see everything out in front of you is that various actions cause your eye to shift about just a little, just enough to cause the image on your retina to shift about enough for the brain to notice.
https://www.aao.org/eye-health/tips-prevention/experiment-se...
Apparently, the brain tends to ignore visual stimuli that don't change over a short period of time, which allows you see "around" the blood vessels passing through the middle of your eye. By closing your eye, and moving a penlight around against your eyelid, you can make the vessels cast a shifting shadow on your retina that makes them visible.
The reason you usually see everything out in front of you is that various actions cause your eye to shift about just a little, just enough to cause the image on your retina to shift about enough for the brain to notice.