Visual hallucinations usually persist longer. Visual illusions are more likely to occur in conditions of dim or poor lighting. Visual illusions may, under circumstances of dim or poor light be normal. Or Visual illusion may be abnormal-a forerunner of visual hallucinations. Hallucinations of sound, auditory hallucinations, can be unformed or formed.
Auditory hallucinations can be frightening, or pleasant.
Unformed auditory hallucinations consist of hearing distorted noises, music, sounds, or voices. Formed auditory hallucinations consist of hearing voices including those of dead, fictional, historical, or living persons. Auditory hallucinations, with the exception of religious experiences, are abnormal. Auditory hallucinations usually occur in schizophrenia. Hallucinations of smell, olfactory hallucinations, are rare in Parkinson disease.