Signs and symptoms
A hallucination is defined as sensory perception in the absence of external stimuli. Hallucinations are different from illusions, or perceptual distortions, which are the misperception of external stimuli. Hallucinations may occur in any of the senses and take on almost any form, which may include simple sensations (such as lights, colors, tastes, and smells) to experiences such as seeing and interacting with fully formed animals and people, hearing voices, and having complex tactile sensations. Hallucinations are generally characterized as being vivid, and uncontrollable.
Auditory hallucinations, particularly experiences of hearing voices, are the most common and often prominent feature of psychosis.
Up to 15% of the general population may experience auditory hallucinations. The prevalence in schizophrenia is generally put around 70%, but may go as high as 98%. During the early 20th century auditory hallucinations were second to visual hallucinations in frequency, but they are now the most common manifestation of schizophrenia, although rates vary throughout cultures and regions. Auditory hallucinations are most commonly intelligible voices. When voices are present, the average number has been estimated at three. Content, like frequency, differs significantly, especially across cultures and demographics. People who experience auditory hallucinations can frequently identify the loudness, location of origin, and may settle on identities for voices. Western cultures are associated with auditory experiences concerning religious content, frequently related to sin. Hallucinations may command a person to do something, which may be dangerous when combined with delusions.
Extracampine hallucinations are auditory hallucinations originating from a particular body part (e.g. a voice coming from a person's knee).
Visual hallucinations occur in roughly a third of people with schizophrenia, although rates as high as 55% are reported. Content frequently involves animate objects, although perceptual abnormalities such as changes in lighting, shading, streaks, or lines may be seen. Visual abnormalities may conflict with proprioceptive information, and visions may include experiences such as the ground tilting. Lilliputian hallucinations are less common in schizophrenia, and occur more frequently in various types of encephalopathy (e.g. Peduncular hallucinosis).
A visceral hallucination, also called a cenesthetic hallucination, is characterized by visceral sensations in the absence of stimuli. Cenesthetic hallucinations may include sensations of burning, or re-arrangement of internal organs.
Psychosis may involve delusional beliefs. Delusions are strong beliefs against the reality, or held despite contradictory evidence. Delusions are necessarily incongruent with societal norms, and some beliefs may constitute a delusion in certain cultures where they impact functioning, while they may be a perfectly normal belief in others. Delusional thinking is relatively common in the general population with around a quarter of people believing they have special power, and a third believing in telepathy. The distinguishing feature between delusional thinking and full blown delusions is the degree with which they impact functioning. Multiple themes are common in delusions, although cultural norms are highly influential (e.g. religious content differing significantly across countries). The most common type of delusion are persecutory delusions, where a person believes that an individual, organization or group is attempting to harm them. Other delusions include delusions of reference (beliefs that a particular stimulus has a special meaning that is directed at the holder of belief), grandiose delusions (delusions that a person has a special power or importance), thought broadcasting (the belief that ones thoughts are audible) and thought insertion (the belief that one's thoughts are not one's own). The DSM-5 characterizes certain delusions as "bizarre" if they are clearly implausible, or are incompatible within the cultural context. The concept of bizarre delusions has been criticized as excessively subjective.
Historically, Karl Jaspers has classified psychotic delusions into primary and secondary types. Primary delusions are defined as arising suddenly and not being comprehensible in terms of normal mental processes, whereas secondary delusions are typically understood as being influenced by the person's background or current situation (e.g., ethnicity; also religious, superstitious, or political beliefs).
Disorganization is split into disorganized speech or thinking, and grossly disorganized motor behavior. Disorganized speech, also called formal thought disorder, is disorganization of thinking that is inferred from speech. Characteristics of disorganized speech include rapidly switching topics, called derailment or loose association; switching to topics that are unrelated, called tangental thinking; incomprehensible speech, called word salad or incoherence. Disorganized motor behavior includes repetitive, odd, or sometimes purposeless movement. Disorganized motor behavior rarely includes catatonia, and although it was a historically prominent symptom, it is rarely seen today. Whether this is due to historically used treatments or the lack thereof is unknown.
Catatonia describes a profoundly agitated state in which the experience of reality is generally considered impaired. There are two primary manifestations of catatonic behavior. The classic presentation is a person who does not move or interact with the world in any way while awake. This type of catatonia presents with waxy flexibility. Waxy flexibility is when someone physically moves part of a catatonic person's body and the person stays in the position even if it is bizarre and otherwise nonfunctional (such as moving a person's arm straight up in the air and the arm staying there).
The other type of catatonia is more of an outward presentation of the profoundly agitated state described above. It involves excessive and purposeless motor behaviour, as well as extreme mental preoccupation that prevents an intact experience of reality. An example is someone walking very fast in circles to the exclusion of anything else with a level of mental preoccupation (meaning not focused on anything relevant to the situation) that was not typical of the person prior to the symptom onset. In both types of catatonia there is generally no reaction to anything that happens outside of them. It is important to distinguish catatonic agitation from severe bipolar mania, although someone could have both.
Negative symptoms include reduced emotional expression, decreased motivation, and reduced spontaneous speech.