A team of Japanese neurologists propose a new method to detect visual hallucinations - the Pareidolia Test.
Pareidolia means perceiving things that aren't there, in random or unrelated stimuli. Uchiyama et al created a set of 25 photos, each of which contains things that kind of look like faces, animals, or other objects... but not really. As you can see, the flowers and the birds look like faces. I can't work out what the leopard and the trees are meant to be, though...
The authors showed the pictures to some patients with dementia. They had one minute to describe what they saw in each image. Compared to healthy controls, patients with Alzheimer's disease did not experience any more pareidolia than controls.
But people with Lewy Body dementia - a disorder in which visual hallucinations and misclassifications are more common than in Alzheimer's - reported seeing numerous faces, people and creatures that weren't there.
They didn't just say that the images "looked like" these things: they actually thought they were pictures of the illusionary objects. This is an interesting test which might help doctors to diagnose visual hallucinations, which are often under-reported by patients. Some degree of pareidolia, especially for faces, is entirely normal, however, as the popularity of Jesus's in snacks shows. When specifically told to expect it, people can even "see" faces in random black and white patterns.
Uchiyama, M., Nishio, Y., Yokoi, K., Hirayama, K., Imamura, T., Shimomura, T., & Mori, E. (2012). Pareidolias: complex visual illusions in dementia with Lewy bodies Brain DOI: 10.1093/brain/aws126