Hallucinations

Hallucinations are principally  visual, auditory, tactile, gustatory.  They are the experience of something being real in which there is no external stimuli to verify the experience.  Misperceptions are differenct, as all illusions.
The significance for psychiatry of hallucinations is that they can have great diagnostic significance.  All hallucinations can occur in brain injuries and in Schizophrenia. Both are considered more organic conditions.  However, visual hallucinations tend to occur more commonly in organic head injuries and with drugs abuse.  In schizophrenia 'bizarre hallucinations' especially auditory hallucinations are often diagnostically helpful.  Alcoholics and drug addicts commonly hear voices inside their head whereas depressive and schizophrenic conditions are associated with voices heard outside the head.    In AA the internal dialogue of addiction has been called the "committee'. It's similiar but to a lesser extent than multiple personality disorder. There is a 'shattering' of perception rather than of ego, per se.
Depressive hallucinations are seen with melancholia and psychotic depression. In psychotic depression the hallucinations are usually auditory though they can be visual with vague black ghosts, the themes being always  negative For example a voice will be heard outside the head calling the person' stupid' , 'no good'.  Sometimes a depressive person can hear two people talking about themselves in a critical way but most often when there are more than one voice talking about the person it's more likely a schizophrenic phenomema. That said, schizophrenia becomes uppermost in the examiner's mind when the person says that the voices aren't really negative but simply commenting on the person or the surroundings. One schizophrenic person said it was like the 'radio' with different commentators talking about different things. In depression the themes are seriously negative but in schizophrenia there can be an absence of emotional tone.
In head injury and especially in drug abuse tactile hallucinations like bugs under the skin are often encountered.
This history helps deliniate these kinds of differences and contribute to the diagnosis.