Though unlikely to be a cause of UFO reports, people here might be interested in Charles Bonnet syndrome.
Some people, usually with failing or severely impaired vision, report visual hallucinations, sometimes "simple" (flashes of light, geometric shapes or grids), sometimes "complex" (structured and detailed images of objects, animals, people).
External Quote:
People with significant vision loss may have vivid recurrent visual hallucinations (fictive visual percepts). One characteristic of these hallucinations is that they usually are "Lilliputian" (hallucinations in which the characters or objects are smaller than normal)
Wikipedia, Visual release hallucinations
https://en.wikipedia.org/wiki/Visual_release_hallucinations
My
impression- before reading the links below- was that the perception of small, often cartoon-like and essentially friendly, or at least benign, figures was a common type of complex Charles Bonnet hallucination, but other sources don't really support that interpretation.
Years ago, I met an elderly lady with recurrent Bonnet hallucinations; she was mildly amused by them- small brightly dressed people and animals- so I expect that shaped my understanding of the subject.
Like most people with Charles Bonnet syndrome, she was fully aware that the hallucinations were just that.
The source is neurological- spontaneous neuronal activity in the visual cortex in the absence of normal input from the visual pathway- but it isn't a mental health condition, and it's not associated with non-vision-related health problems.
Causes of visual impairment that might cause Charles Bonnet Syndrome include macular degeneration and cataracts.
Cataracts are readily treatable in most people; improved vision ends Bonnet hallucinations.
(UK) National Health Service,
Charles Bonnet syndrome https://www.nhs.uk/conditions/charles-bonnet-syndrome/,
"Effect of cataract surgery on cognition, mood, and visual hallucinations in older adults",
Journal of Cataract and Refractive Surgery 41 (6), J.M. Jefferis, M.P. Clarke, J.P. Taylor
https://pubmed.ncbi.nlm.nih.gov/26096519/;
"Effect of cataract surgery on visual hallucinations in older adults" [letter], C.S. Tan, Kai, Cheong
et al.,
Journal of Cataract & Refractive Surgery 41 (10), h
ttps://journals.lww.com/jcrs/Fulltext/2015/10000/Effect_of_cataract_surgery_on_visual.48.aspx
External Quote:
One thing that is certain is that hallucinations do not mean the person is mentally ill. However bizarre, frightening or funny their content, Charles Bonnet hallucinations are no more than a normal brain's response to reduced visual input.
While they may be an inconvenience, they are not a cause for concern. If you find your hallucinations upsetting, talk to your doctor or ophthalmologist about the problem.
Macular Society,
Charles Bonnet Syndrome (CBS),
https://www.macularsociety.org/macular-disease/macular-conditions/charles-bonnet-syndrome/
Again, CBS is unlikely to be a cause of UFO/ Nessie/ Sasquatch etc. sightings, as the experiencers usually have very poor vision which is known both to themselves and others, and (importantly) the experiencer is almost always aware of the illusory nature of the hallucinations.
Can't help but wonder if sometimes very elderly residents with poor vision in e.g. care homes, maybe with undiagnosed CBS, might report the odd strange sighting to carers, perhaps starting a ghost story. Pure speculation though.
There is a transitory, possibly related phenomena known as "The Prisoner's Cinema" where some people, in conditions of darkness for protracted periods, have reported visual hallucinations,
https://en.wikipedia.org/wiki/Prisoner's_cinema
I've attached a PDF, "Understanding Charles Bonnet syndrome", 2024, Royal National Institute of Blind People.
It states that (some) children with visual loss also experience CBS.
________________________________________________________________________________________________________________________
"Yesterday, upon the stair, I met a man who wasn't there..."
Huh! I just learned that is from a poem called
"Antigonish" by Hughes Mearns. I only knew of it from the 2003 psychological thriller
"Identity" which, without giving spoilers, deals with some psychological issues and the perception of reality.
I first heard it in an episode of
Sapphire and Steel, 1979-1982. Joanna Lumley and David McCallum played the eponymous temporal agents, trying to fix "leaks" in time, where (unpleasant) events from some previous era intruded into the present.
Like several other British SF series aimed at children around that time- along with public information films, and some dramas broadcast for family viewing (
Survivors;
The Nightmare Man)- there seemed to be a concerted effort to scare the nation's children sh*tless.