A neurological case study was recently reported on visual hallucinations in photographs. I found this particularly fascinating. Two Parkinson's patients with severe visual hallucinations and delusions were shown photographic evidence proving that what they were seeing wasn't actually there. Strangely enough, the patients' hallucinations extended into the photographic realm, and they reported seeing the hallucinations there, too. Utterly bizarre.
This is apparently the first reported visual hallucination that persisted into photographs of the scene where the hallucinations/delusions were occuring. The researchers referred to this phenomenon as "delusional hallucinations" or "delusional illusions." They discovered this oddity by taking photographs of the scene in which the hallucinations were occuring, then showing them to the patients in order to prove nothing was actually there. But the patients saw their hallucinations in the scene anyway.
It's almost as if we were to show UFO or Bigfoot believers photographs or videos of the scenes in which they saw something, and proved no aliens or Bigfoots were there, the viewers would STILL report seeing the same thing! Of course this assumes the viewers in question were truly hallucinating, which is probably not the case for the majority of supernatural reports. Most supernatural reports appear to usually be misidentifications or misunderstandings of a scene (such as black bears being misidentified as Sasquatches, and aircraft or meteorological or astronomical phenomena misidentified as alien-driving UFOs).
Back to the case report. Something about the real-world scenes in which the hallucinations were occurring was contributing or in some sense causing/driving the hallucinations (it might have been a particular tree, or flower bed arrangement in a yard, or whatever; it's not clear exactly). And apparently the same stimuli from the scene, when portrayed from a photograph, was enough to cause the same hallucinations to bleed over from the real-world into the photo. Identifying such visual features that were contributing to the hallucinations might now be experimentally possible given these results, and allow for deep experimental study of visual hallucinations and delusions. I'm not aware if this particular phenomenon is limited to only delusions/hallucinations in Parkinson's disease, or if such reports have been made or are even possible with other neurological syndromes.
This is a remarkable testament into how the brain constructs our perception of the world around us, using (sometimes sub-par) visual information to construct a "best guess" about what is out there in the world. It also speaks to the power of photographs: to some part of ourselves, a few splotches of light placed here-or-there in the right arrangement elicits powerful feelings, and seems undeniably real on some level. We keep ourselves surrounded by pictures of our loved ones, hanging on our walls, on our desks, in our phones, and stuffed in our wallets and purses. Even for persons long departed, a photo has the power to let us feel their presence once more.