*Hallucination is a false sensory perception not associated with real external stimuli.
*The sensory system (eg:auditory, visual, taste, olfactory or tactile ) is involved.
*It occurs in inner subjective space (rather than as imagined) or experienced as originating in the outside world.
*It is independent of the will of the observer.
*It is as vivid as true sense perception and may be frightening.
*HYPNAGOGIC hallucinations (occuring while falling asleep) and HYPNOPOMPIC hallucinations (occuring while awakening from sleep) are generally considered non-pathological and are common in narcolepsy.
*Gustatory hallucinations (unpleasant taste) occur in uncinate seizure.
*Tactile (haptic) hallucinations are formication are seen in cocaine abuse.
*Release hallucinations are due to sensory pathway disease as optic neuritis, otosclerosis etc.
(Hallucinations that develop after partial loss of sight or hearing, and represent images or sounds formed from memory traces rather than present sensory input. They are called "release" hallucinations because they would ordinarily be blocked by incoming sensory data.)
*Most common non-organic hallucination is auditory.
*Most common organic hallucination is visual.