Hallucinations A hallucination is defined as sensory perception in the absence of external stimuli. Hallucinations are different from illusions and perceptual distortions, which are the misperception of external stimuli. Hallucinations may occur in any of the senses and take on almost any form. They may consist of simple sensations (such as lights, colors, sounds, tastes, or smells) or more detailed experiences (such as seeing and interacting with 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<unk> . Up to 15% of the general population may experience auditory hallucinations (though not all are due to<unk> . The prevalence of auditory hallucinations in patients with schizophrenia is generally put around 70%, but may go as high as 98%. Reported prevalence in bipolar disorder ranges between 11% and 68%. 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 between 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 potentially dangerous when combined with delusions. So-called "minor hallucinations", such as extracampine hallucinations, or false perceptions of people or movement occurring outside of one's visual field, frequently occur in neurocognitive disorders, such as Parkinson's disease. Visual hallucinations occur in roughly a third of people with schizophrenia, although rates as high as 55% are reported. The prevalence in bipolar disorder is around 15%. Content commonly 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 are more common in various types of encephalopathy such as 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. Delusions<unk>  may involve delusional beliefs. A delusion is a fixed, false idiosyncratic belief, which does not change even when presented with incontrovertible evidence to the contrary. Delusions are context- and culture-dependent: a belief which inhibits critical functioning and is widely considered delusional in one population may be common (and even adaptive) in another, or in the same population at a later time. Since normative views may contradict available evidence, a belief need not contravene cultural standards in order to be considered delusional. Prevalence in schizophrenia is generally considered at least 90%, and around 50% in bipolar disorder. The DSM-5 characterizes certain delusions as "bizarre" if they are clearly implausible, or are incompatible with the surrounding cultural context. The concept of bizarre delusions has many criticisms, the most prominent being judging its presence is not highly reliable even among trained individuals. A delusion may involve diverse thematic content. The most common type is a persecutory delusion, in which a person believes that an entity seeks to harm them. Others include delusions of reference (the belief that some element of one's experience represents a deliberate and specific act by or message from some other entity), delusions of grandeur (the belief that one possesses special power or influence beyond one's actual limits), thought broadcasting (the belief that one's thoughts are audible) and thought insertion (the belief that one's thoughts are not one's own). The subject matter of delusions seems to reflect the current culture in a particular time and location. For example, in the US, during the early 1900s syphilis was a common topic, during the second world war Germany, during the cold war communists, and in recent years technology has been a focus. Some psychologists, such as those who practice the Open Dialogue method, believe that the content of<unk>  represents an underlying thought process that may, in part, be responsible for<unk> , though the accepted medical position is that<unk>  is due to a brain disorder. Historically, Karl Jaspers 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 of speech/thought or behavior Disorganization is split into disorganized speech (or thought), and grossly disorganized motor behavior. Disorganized speech or thought, 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 tangential 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 an 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 Negative symptoms include reduced emotional expression (flat affect), decreased motivation (avolition), and reduced spontaneous speech (poverty of speech, alogia). Afflicted individuals lack interest and spontaneity, and have the inability to feel pleasure (anhedonia).<unk>  in adolescents<unk>  is rare in adolescents. Young people who have<unk>  may have trouble connecting with the world around them and may experience hallucinations and/or delusions. Adolescents with<unk>  may also have cognitive deficits that may make it harder for the youth to socialize and work. Potential impairments include the speed of mental processing, ability to focus without getting distracted (limited attention span), and deficits in verbal memory. If an adolescent is experiencing<unk> , they most likely have comorbidity meaning they could have multiple mental illnesses. Because of this, it can be difficult to determine if it is<unk>  or autism spectrum disorder, social or generalized anxiety disorder, or obsessive-compulsive disorder.
Psychosis
Anti-NMDA receptor encephalitis
Depression 
Dissociative identity disorder
Psychotic depression
Phobia
Depression and Anxiety
Collective depression