What is a hallucination?

A hallucination is the experience of a sensory perception despite the absence of an external stimulus. Patients find these experiences very realistic, and these can occur with any senses- visual, auditory, gustatory, olfactory or tactile. Generally speaking, a hallucination can  be defined only when the person was conscious during the episode.


Auditory hallucinations (AH) are the most common type of hallucination in people with conditions such as schizophrenia and is normally described as ‘hearing voices in the head’. The voices can be complimentary, but are often critical with negative content. Sometimes the voices might be talking about the person, or even speak coherently to the person, giving out commands and engaging the person in conversation. One, or many voices can be heard at a time; most of the time the person is unable to control the onset or disappearance of the voices, leaving them feeling very helpless and frustrated.

The process that underlies AH remains largely unknown, AH is commonly found in persons with schizophrenia, with a prevalence of 70% in that population (1). 8-15% of AH have been estimated to occur without any neuropsychiatric illnesses (2); these may have been triggered by traumatic events, drug intoxication or other diseases such as visual impairment. Whilst simpler AH such as the inability to distinguish between noise and a tune are thought to be related to other brain pathologies, complex AH with organised and structured sentences are more likely to be due to schizophrenia.


According to the Dialogical Self Theory, our brain ‘rehearses’ conversations unconsciously. When there is a problem in our thought processing, we may mistake our own thoughts as signals from the surroundings thus hallucinations are heard. Whereas the Attribution Theory however suggests that patients attribute all sounds heard as someone else’s.  


Contrary to AH, visual hallucinations (VH) are less prevalent in persons with schizophrenia.  VH are seeing things that do not exist; these ‘things’ can be an object, a person or a picture, e.g. when the person hallucinates, (s)he may see a non-existent person whom no one else can see. Like AH, VH can occur as a result of schizophrenia as well as other brain pathologies; simpler VH such as colours and shapes are related to the latter, ranges from encephalitis, meningitis to hypoglycaemia and low levels of oxygen in the blood. More complex VH are more closely related to delirium and schizophrenia.

Other patient may have tactile hallucination, felt that there are moving objects on their body such as insects, or their organs moving within their body and being touched.


As these hallucinations are very realistic for the patient, if your loved one is experiencing hallucinations, do not deny their existence, instead listen to them and give appropriate responses. If you wish to find out how to manage hallucinations, please click here.

(1) Tracy, D.K.; Shergill, S.S.    Mechanisms Underlying Auditory Hallucinations—Understanding Perception without Stimulus. Brain Sci. 2013, 3, 642-669.

(2) L.J. Johns, J.Y. Nazroo, P. Bebbington, E. Kuipers. Occurrence of hallucinatory experiences in a community sample and ethnic variations. Br. J. Psychiatry, 180 (2002), pp. 174–178