I have several times in my life experienced migraine hallucinations. I call them that because they look exactly like what other people report under that name.
I'll come back to those.
If I look at someone, and hold up my hand so as to block my view of their head, I do not experience looking at a headless person. I experience looking at a normal person, whose head I cannot see, because there is something else in the way.
Why is this? One can instantly talk about Bayesian estimation, prior experience, training of neural nets, constant conjunction, and so on. However, a real explanation must also account for situations in which this filling-in does not occur. One ordinary example is the pictures here. I see these as headless men, not ordinary men whose heads I cannot see.
Migraine hallucinations provide a more interesting example. If you've ever had one, you might already know what I'm going to say, but I do not know if this experience is the same for everyone.
If I superimpose the hallucination on someone's head, they seem to have no head. I don't mean that I cannot see their head (although indeed I can't), but that I seem to be looking at a headless person. If I superimpose it on a part of their head, it is as if that part does not exist. Whatever the blind spot covers, my brain does not fill it in. Whatever my hand covers, my brain does fill in, not at the level of the image (I don't confabulate an image of their face), but at some higher level. I know in both cases that they have a head. But at some level below knowing, the experience in one case is that they have no head, and in the other, that they do. My knowledge that they have a head does nothing to alter the sensation that they do not.
It is quite disconcerting to look at myself in a mirror and see half my head missing.
Those who have never had such hallucinations might try experimenting with their ordinary blind spots. I am not sure it will be the same. The brain has had more practice filling those in, and does not have to contend with the jaggies.
From this I cannot draw out much in the way of conclusions about vision and the brain, but it provides an interesting experience of the separation between two levels of abstraction. When we look at the world and see comprehensible objects in it, our brain did that before it ever came into our subjective experience. When the mechanism develops a fault, it presents conclusions that we know to be false, yet still experience.
This presumably applies to all our senses, including that of introspection.