Delusions are typical symptom of various mental disorders, such as schizophrenia, delusional disorder, schizoaffective disorder, bipolar disorder, and schizophreniform disorder. Hallucinations, on the other hand, tend to appear only in people with schizophrenia, bipolar disorder with psychotic features, or other psychotic disorders.
Delusions
Delusions are false or erroneous beliefs that usually involve a misinterpretation of perceptions or experiences. Their content may include a variety of themes (e.g., persecutory, referential, somatic, religious, or grandiose).
Persecutory delusions are most common; the person believes he or she is being tormented, followed, tricked, spied on, or ridiculed. Referential delusions are also common; the person believes that certain gestures, comments, passages from books, newspapers, song lyrics, or other environmental cues are specifically directed at him or her.
The distinction between a delusion and a strongly held idea is sometimes difficult to make and depends in part on the degree of conviction with which the belief is held despite clear contradictory evidence regarding its veracity.
Although bizarre delusions are considered especially characteristic of schizophrenia, “bizarreness” may be difficult to judge, especially across different cultures. Delusions are deemed bizarre if they are clearly implausible and not understandable and do not derive from ordinary life experiences.
An example of a bizarre delusion is a person’s belief that a stranger has removed his or her internal organs and has replaced them with someone else’s organs without leaving any wounds or scars.
Delusions that express a loss of control over mind or body are generally considered bizarre. These include: a person’s belief that his or her thoughts have been taken away by some outside force (“thought withdrawal”); that alien thoughts have been put into his or her mind (“thought insertion”); or that his or her body or actions are being acted on or manipulated by some outside force (“delusions of control”).
An example of a non-bizarre delusion is a person’s false belief that he or she is under surveillance by the police.
Hallucinations
Hallucinations may occur in any sensory modality (e.g., auditory, visual, olfactory, gustatory, and tactile), but auditory hallucinations are by far the most common. Auditory hallucinations are usually experienced as voices, whether familiar or unfamiliar, which are perceived as distinct from the person’s own thoughts.
The hallucinations must occur in the context of a clear sensorium; those that occur while falling asleep (hypnagogic) or waking up (hypnopompic) are considered to be within the range of normal experience.
Isolated experiences of hearing one’s name called or experiences that lack the quality of an external percept (e.g., a humming in one’s head) should also not be considered as symptomatic of Schizophrenia or any other Psychotic Disorder.
Hallucinations may be a normal part of religious experience in certain cultural contexts. Certain types of auditory hallucinations (i.e., two or more voices conversing with one another or voices maintaining a running commentary on the person’s thoughts or behavior) have been considered particularly characteristic of Schizophrenia.
Source: American Psychiatric Association