But, in a variant of Parkinson disease, Dementia with Lewy Bodies, hallucinations may appear independently. A visual hallucination consists of a perception of an inanimate object, or an animal, or a person without any of the above being present. Auditory hallucinations are usually a symptom of schizophrenia. Illusions differ from hallucinations. Or seeing a person's form in the shape of a chair.
Illusions are similar to mirages.
Hallucinations, with the exception of religious experiences, are usually abnormal as the circumstances causing them are usually abnormal. Such circumstances include changes in the brain related to electrolyte disturbances, high fevers, liver or kidney failure or certain drugs. Illusions, however, may be normal as the circumstances causing them may be normal. Such circumstances include poor lighting or, paradoxically, bright sunlight. Such hallucinations can usually be ignored.