Yes, once again, this is about my father.
Brief back-story for those who have not read the previous of my father's encounters: my father lives in a small town in Indiana in a cute, two story, 150 year old house. It is in a state of renovation currently. My dad has a history of being followed by a darker being that seems to enjoy terrifying him every now and again since he was young. A few months ago, a woman starting appearing in his house. She was very transparent at that time and seems to be harmless - seems to be wearing turn-of-the-century clothing.
My father was lying in bed around three weeks ago, reading. My step-mother was in Scotland visiting her family, and the cats were curled up with my father. He hears footsteps on the stairs (which are becoming a daily thing) and decided to keep reading and just ignore them. The footsteps continue the few feet to his bedroom door and he sees a shape in the corner of his eye. He looks up and sees a full figure of a woman, nearly solid. She is wearing a dark, turn-of-the-century style dress and has brown hair, in a bun. She looks to be middle aged. Other than her clothing, she looked quite normal. What he found to be strange is that she was looking in the room, but she was not looking at him. She seemed to be focused behind him.
After about ten seconds, she just dissipates. The cats heard her walk up and were staring at her as well, though they did not seem to react with any sort of fear. Other than being surprised, my dad was not frightened of her. She does not give off any sort of menacing vibe.
This is the first time my father was able to see color and her face.
Fast forward to when my stepmother returns from Scotland, about a week later. They are laying in bed with the cats around 11 at night. They are both reading and relaxing when footsteps start coming up the stairs. The steps, like last time, walk to the door and stop, but there was no apparition involved this time. My stepmother, who previously thought my dad was a bit loony for thinking the house had a visitor, looks at my dad and asks if he heard it too. I believe the direct quote from my father was, "I told you so."
The activity definitely seems to be building in my father's house, but it is not disruptive and apparently the large bangs that used to happen frequently do not happen often anymore.
My sister named her Victoria, mostly because of how her clothing style was described.
I believe that I have convinced by father that next time she comes to visit, he should attempt to have a conversation with her since the activity seems to be centered around where he happens to be. He is also a talented amateur photographer and I may be able convince him to take some pictures, as long as it seems safe to do so. She seems to be very peaceful, despite the early fears of her being somehow connected to the other entity my father has dealt with. We cannot currently figure out if she is residual or intelligent, because both seem to fit. She does seem to purposefully wander around where my father happens to be.
I will continue to give updates on my father's new housemate as I receive them. I am quite happy that my stepmother was able to validate my father's experiences this time. I am looking forward to my next trip to my father's house.
Your brain while hallucinating is indeed strange. It will pick up what it's processing and act as if it's completely normal, which is usually a good thing- the ability to store what is out of place is a good way to create a valid diagnosis. The visual areas of the brain will generate the hallucination on it's own, which is why those lobes are so active during a hallucination. This is the same with auditory hallucinations- the temporal lobes will be generating the same sound that they are processing. It's pretty redundant. What makes them so tricky is the brain thinks it is experiencing these things instead of generating them- which is why hallucinations are often believed to be real.
They have been trying to precisely map it for a few decades. This study: http://cercor.oxfordjournals.org/content/12/12/1331.full is really interesting. Their comparison of the schizophrenic brain to a "normal" brain is actually very intriguing. The structural differences are very interesting to read up on, as well as the chemical differences.
This one has a few good studies: http://www.julianjaynes.org/related-articles_neuroimaging.php
What is also sort of funny is that an MRI can generate hallucinations in people with no history because of the high magnetic fields (if you combine that with rapid movement). It's rare, but it can cause that "frying bacon" effect.
Now onto the paranormal: I am not sure that I believe that the two share any sort of connection with each other, but the idea is interesting. There is a pretty common paranormal theory that children are able to see spirits because of how their brain is still forming and their view of the world is therefore much different than a mature brain. If this theory is actually true, then those with severe mental illnesses could possibly be more in-tune with a world that is just outside of our normal perceptions. This would be even more interesting to think about in functional schizophrenic disorders. Before modern medicine, it was widely believed that illnesses like schizophrenia and dissociative identity disorders were the work of demonic activity (hence the popularity of exorcisms and some very famous deaths because of them).
Back on topic, again, if you combine a belief in the paranormal when looking at mental illness, the two can combine themselves fairly easily. In the Mr. Adam's case, having a sort of spirit guide fighting off the "bad" delusions could really be more symbolic of the good part of the patient fighting off the part that it perceives as bad, manifesting itself as a hallucination of someone that the patient trusted. Or, it could literally be a spirit trying to help someone through one of the most difficult mental illness. Personally, I really like the last idea. Professionally, the first one is the most likely. The fact that multiple people profess to have witnessed the same thing gives the spirit idea a little more credit, seeming how shared delusions are incredibly rare - and generally unproven- and usually occur only in times of intense stress, which is not the case in that story.
Generally, what I am attempting to get at is the science and psychology cannot explain everything, even though it can explain a lot. It figured out sleep paralysis, psychosis and delusions and most people fall into certain categories of "sanity". I think there could be some sort of ties into brain development and an ability to see things that others typically cannot- real things. The problem becomes the lack of evidence to support that, which ties in with the fact that hallucinations already appear real- how can someone separate the two without some sort of external witness to the phenomenon? If a spirit was going to follow someone, it would make sense to follow someone who actually had the capacity to see them.
There are many ties between mental disorders and a paranormal belief, but most of the studies have been dismissed as pseudoscience with a bias for the paranormal (though those who dismiss it fully also have a bias for science, so really, everyone is biased). There have also been studies dismissing a paranormal belief by finding correlations between depressive disorders and the belief: https://www.gold.ac.uk/media/1995-thalbourne-paranormal.pdf (that one is really short, it's just a summary, for some reason I could not find the full version). About a quarter of Americans believe they have seen a ghost, while others surveys have only 60% believing that they have never seen a ghost. A complete dismissal of a possibility is not logical. While a lot of the evidence for the paranormal can be explained away, that many people having visual and even shared experiences in seeing something that should not be there should be an area that more people actually study instead of shrug off. After all, ancient monks got meditation correct without having any way to prove it: http://www.news.wisc.edu/13890
It's obviously not sane to think that anyone who witnesses a paranormal event is hallucinating, because most people will go their entire lives without hallucinating fully (dream disturbances and reactions to medicines not being counted in this equation). By that very logic, it's not sane to believe that all hallucinations are created solely by the mind.
I guess all-in-all my only real reply would be: Anything is possible.
That got sort of rambly. I hope that sort of answered the question.