Hallucination is an error in perception in which a person sees, hears, feels, or smells something that is not there. For example, a person may hear voices when no one is speaking, or feel something touch him in an empty room. Some people have quick passing hallucination, particularly when falling asleep or waking up and this is considered normal. Hallucinations and delusions that persist or recur may be a sign of mental illness.

Types of Normal Hallucination :

• Hypnagogic hallucination - happens when a person is falling asleep. Usually last from seconds to minutes.
• Hypnopompic hallucination - happen when one is waking up.

Causes of Hallucination:

• It may be induced by drugs (hallucinogenic drugs) or hypnosis
• It may be due to lack of sleep
• Depression
• Mental Illness

Hallucination is sometimes confused with Illusion and Delusion.

• Illusion - something not real or imaginary. An example is, a shadow of a curtain taken for a person.

• Delusion - is a mistaken belief that cannot be corrected by reasoning. An example is, a person may think that a dear friend is trying to kill him.