Hallucinations are perceptions which take place in the absence of a stimulus from the peripheral or sensory nervous system.
They may be classified in a variety of different ways (this is an incomplete list):
1)by sensory modality
a) auditory: these are most common, and may be perceived as voices speaking or mumbling; musical sounds; or other more cacophonous sounds
b) visual: these can occur more commonly in delirious states or medical illnesses affecting the brain. Many people experience normal, but unsettling, visual hallucinations, just when falling asleep or waking up.
c) tactile: these are most common in chemical intoxication syndromes, such as with cocaine.
d) olfactory: more common in medical illness
2) by positionality
-when describing hallucinated voices, if the voices are perceived to originate inside the head, or to not have any perceived origin, then they could be called "pseudohallucinations." If the voices are perceived to originate from a particular place, such as from the ceiling or from across the room, then they could be called "hallucinations" or "true hallucinations." This terminology has been used to distinguish between the hallucinations in schizophrenia and psychotic mood disorders (which are typically "true hallucinations") and those experienced in non-psychotic disorders (pseudohallucinations are more typically--though not invariably--associated with dissociative disorders, borderline personality, or PTSD).
3) by insight
An individual experiencing a "psychotic hallucination" will attribute the phenomenon to stimuli outside of the brain. An individual experiencing a "non-psychotic hallucination" will attribute the phenomenon to his or her own brain activity, and recognize the absence of an external stimulus to account for the experience. In most cases, "insight" fluctuates on a continuum, and many individuals experiencing hallucinations will have some intellectual understanding of their perceptions being hallucinatory, but still feel on a visceral level that the perceptions are "real."
4) by character
Voices in particular can be described in a variety of ways. So-called "first rank symptoms of schizophrenia" include hallucinated voices which comment on a person's behavior, or include several voices which converse with each other.
The quality of the voice can vary, with harsh, angry, critical tones more common in psychotic depression, and neutral emotionality more common in schizophrenic states.
--all of these above descriptions are incomplete, and associations between one type of hallucination and a specific "diagnosis" are imperfect. A great deal of variation exists--
It is probably true that some hallucinations are factitious (i.e. the person is not actually hallucinating, despite claiming to), but of course this would be virtually impossible to prove. Something like functional brain imaging might be an interesting, though impractical, tool, to examine this phenomenon. People with psychotic disorders or borderline personality might at times describe factitious hallucinatory phenomena in order to communicate emotional distress or need to caregivers. Or sometimes the phenomena may convey some type of figurative meaning. The motivation to do this might not always be conscious.
There are a variety of ways to treat hallucinations.
In my opinion, the single most effective treatment is an antipsychotic medication. Hallucinations due to almost any cause are likely to diminish with antipsychotic medication treatment.
There is evolving evidence that CBT and other psychotherapy can help with hallucinations. Here are some references:
Some individuals may not be bothered by their hallucinations. In this case, it may sometimes be more the physician's agenda than the patient's to "treat" the symptom. Yet, it is probably true that active hallucinations in psychotic disorders are harbingers of other worsening symptoms, so it may be important to treat the symptom early, even if it is not troublesome.
Other types of behavioral tactics can help, including listening to music, wearing ear plugs, other distractions, etc. In dealing with pseudohallucinations or non-psychotic hallucinations, "mindfulness" exercises may be quite important. A well-boundaried psychodynamically-oriented therapy structure could be very helpful for non-psychotic hallucinations or pseudohallucinations associated with borderline personality dynamics or PTSD. Care would need to be taken, in these cases, not to focus excessively or "deeply" on the hallucinations, particularly without the patient's clear consent, since such a dialog could intensify the symptoms.