Thought disorder (TD) or formal thought disorder (FTD) refers to disorganized thinking as evidenced by disorganized speech. Specific thought disorders include derailment, poverty of speech, tangentiality, illogicality, perseveration, and thought blocking.

Psychiatrists consider formal thought disorder as being one of two types of disordered thinking, with the other type being delusions. The latter involves "content" while the former involves "form". Although the term "thought disorder" can refer to either type, in common parlance it refers most often to a disorder of thought "form" also known as formal thought disorder.

Eugen Bleuler, who named schizophrenia, held that thought disorder was its defining characteristic. However, formal thought disorder is not unique to schizophrenia or psychosis. It is often a symptom of mania, and less often it can be present in other mental disorders such as depression. Clanging or echolalia may be present in Tourette syndrome. Patients with a clouded consciousness, like that found in delirium, also have a formal thought disorder.

However, there is a clinical difference between these two groups. Those with schizophrenia or psychosis are less likely to demonstrate awareness or concern about the disordered thinking. Clayton and Winokur have suggested that this results from a fundamental inability to use the same type of Aristotelian logic as others. On the other hand, patients with a clouded consciousness, referred to as "organic" patients, usually do demonstrate awareness and concern, and complain about being "confused" or "unable to think straight"; Clayton and Winokur suggest that this is because their thought disorder results, instead, from various cognitive deficits.

In psychiatry, derailment (also loosening of association, asyndesis, asyndetic thinking, knight's move thinking, or entgleisen) is a thought disorder characterized by discourse consisting of a sequence of unrelated or only remotely related ideas. The frame of reference often changes from one sentence to the next.

In a mild manifestation, this thought disorder is characterized by slippage of ideas further and further from the point of a discussion. Derailment can often be manifestly caused by intense emotions such as euphoria or hysteria. Some of the synonyms given above ("loosening of association", "asyndetic thinking") are used by some authors to refer just to a "loss of goal": discourse that sets off on a particular idea, wanders off and never returns to it. A related term is tangentiality—it refers to off-the-point, oblique or irrelevant answers given to questions. In some studies on creativity, "knight's move thinking", while it describes a similarly loose association of ideas, is not considered a mental disorder or the hallmark of one; it is sometimes used as a synonym for lateral thinking.

In psychology and psychiatry, clanging refers to a mode of speech characterized by association of words based upon sound rather than concepts. For example, this may include compulsive rhyming or alliteration without apparent logical connection between words. This is associated with the irregular thinking apparent in psychotic mental illnesses (e.g. mania and schizophrenia). Gustav Aschaffenburg found that manic individuals generated these "clang-associations" roughly 10-50 times more than non-manic individuals. Aschaffenburg also found that the frequency of these associations increased for all individuals as they became more fatigued.

Clanging refers specifically to behavior that is situationally inappropriate. While a poet rhyming is not evidence of mental illness, disorganized speech that impedes the patient's ability to communicate is a disorder in itself, often seen in schizophrenia.

Once it has been positively identified, pharmacotherapy follows. Antipsychotic drugs are the frontrunners in treatment for Fregoli and other DMSs. In addition to antipsychotics, anticonvulsants and antidepressants are also prescribed in some treatment courses. If a Fregoli patient has other psychological disorders, treatment often results in the use of trifluoperazine.

The concept of thought disorder has been criticized as being based on circular or incoherent definitions. For example, thought disorder is inferred from disordered speech, based on the assumption that disordered speech arises because of disordered thought. Incoherence, or word salad, refers to speech that is unconnected and conveys no meaning to the listener.

Furthermore, although thought disorder is typically associated with psychosis, similar phenomena can appear in different disorders, potentially leading to misdiagnosis—for example, in the case of incomplete yet potentially fruitful thought processes.

It has been suggested that individuals with autism spectrum disorders (ASD) display language disturbances like those found in schizophrenia; a 2008 study found that children and adolescents with ASD showed significantly more illogical thinking and loose associations than control subjects. The illogical thinking was related to cognitive functioning and executive control; the loose associations were related to communication symptoms and to parent reports of stress and anxiety.

Word salad can be generated by a computer program for entertainment purposes by inserting randomly chosen words of the same type (nouns, adjectives, etc.) into template sentences with missing words, a game similar to Mad Libs. The video game company Maxis, in their seminal SimCity 2000, used this technique to create an in-game "newspaper" for entertainment; the columns were composed by taking a vague story-structure, and using randomization, inserted various nouns, adjectives, and verbs to generate seemingly unique stories.

Another way of generating meaningless text is mojibake, also called "Buchstabensalat" ("letter salad") in German, in which an assortment of seemingly random text is generated through character encoding incompatibility in which one set of characters are replaced by another, though the effect is more effective in languages where each character represents a word, such as Chinese, than a letter.

More serious attempts to produce nonsense automatically stem from Claude Shannon's seminal paper "A Mathematical Theory of Communication" from 1948

where progressively more convincing nonsense is generated first by choosing letters and spaces randomly, then according to the frequency with which each character appears in some sample of text, then respecting the likelihood that the chosen letter appears after the preceding one or two in the sample text, and then applying similar techniques to whole words. Its most convincing nonsense is generated by second-order word approximation, in which words are chosen by a random function weighted to the likeliness that each word follows the preceding one in normal text:

THE HEAD AND IN FRONTAL ATTACK ON AN ENGLISH WRITER THAT THE CHAR-

ACTER OF THIS POINT IS THEREFORE ANOTHER METHOD FOR THE LETTERS THAT

THE TIME OF WHO EVER TOLD THE PROBLEM FOR AN UNEXPECTED.

Markov chains can be used to generate random but somewhat human-looking sentences. This is used in some chat-bots, especially on IRC-networks.

Nonsensical phrasing can also be generated for more malicious reasons, such as the Bayesian poisoning used to counter Bayesian spam filters by using a string of words which have a high probability of being collocated in English, but with no concern for whether the sentence makes sense grammatically or logically.

Treatment is similar to that for other forms of obsessive–compulsive disorder. Exposure and response prevention (ERP), a form of behavior therapy, is widely used for OCD in general and may be promising for scrupulosity in particular. ERP is based on the idea that deliberate repeated exposure to obsessional stimuli lessens anxiety, and that avoiding rituals lowers the urge to behave compulsively. For example, with ERP a person obsessed by blasphemous thoughts while reading the Bible would practice reading the Bible. However, ERP is considerably harder to implement than with other disorders, because scrupulosity often involves spiritual issues that are not specific situations and objects. For example, ERP is not appropriate for a man obsessed by feelings that God has rejected and is punishing him. Cognitive therapy may be appropriate when ERP is not feasible. Other therapy strategies include noting contradictions between the compulsive behaviors and moral or religious teachings, and informing individuals that for centuries religious figures have suggested strategies similar to ERP. Religious counseling may be an additional way to readjust beliefs associated with the disorder, though it may also stimulate greater anxiety.

Little evidence is available on the use of medications to treat scrupulosity. Although serotonergic medications are often used to treat OCD, studies of pharmacologic treatment of scrupulosity in particular have produced so few results that even tentative recommendations cannot be made.

Treatment of scrupulosity in children has not been investigated to the extent it has been studied in adults, and one of the factors that makes the treatment difficult is the fine line the therapist must walk between engaging and offending the client.

The prevalence of scrupulosity is speculative. Available data do not permit reliable estimates, and available analyses mostly disregard associations with age or with gender, and have not reliably addressed associations with geography or ethnicity. Available data suggest that the prevalence of obsessive–compulsive disorder does not differ by culture, except where prevalence rates differ for all psychiatric disorders. No association between OCD and depth of religious beliefs has been demonstrated, although data are scarce. There are large regional differences in the percentage of OCD patients who have religious obsessions or compulsions, ranging from 0–7% in countries like the U.K. and Singapore, to 40–60% in traditional Muslim and orthodox Jewish populations.

Communication deviance (CD) occurs when a speaker fails to effectively communicate meaning to their listener with confusing speech patterns or illogical patterns. These disturbances can range from vague linguistic references, contradictory statements to more encompassing non-verbal problems at the level of turn-taking. The term was originally introduced by Wynne and Singer in 1963 to describe a communication style found among parents who had children with schizophrenia. A recent meta-analysis reported that communication deviance is highly prevalent in parents of patients diagnosed with schizophrenia and adoption studies have reported significant associations between CD in the parent and thought disorder in the offspring, however, the mechanisms by which CD impacts on the offspring's cognition are still unknown.

The research of psychiatrists and psychoanalysts Lyman Wynne and Theodore Lidz on communication deviance and roles (e.g., pseudo-mutuality, pseudo-hostility, schism and skew) in families of people with schizophrenia also became influential with "systems-communications"-oriented theorists and therapists.

Word salad is a "confused or unintelligible mixture of seemingly random words and phrases", most often used to describe a symptom of a neurological or mental disorder. The words may or may not be grammatically correct, but are semantically confused to the point that the listener cannot extract any meaning from them. The term is often used in psychiatry as well as in theoretical linguistics to describe a type of grammatical acceptability judgment by native speakers, and in computer programming to describe textual randomization.

This form of schizophrenia is typically associated with early onset (often between the ages of 15 and 25 years) and is thought to have a poor prognosis because of the rapid development of negative symptoms and decline in social functioning.

Use of electroconvulsive therapy has been proposed; however, the effectiveness after treatment is in question.

Disorganized schizophrenia, also known as hebephrenia or hebephrenic schizophrenia, is a subtype of schizophrenia, although it is not recognized in the latest version of the "Diagnostic and Statistical Manual of Mental Disorders". It's recognized only in the 10th revision of the International Statistical Classification of Diseases and Related Health Problems (ICD-10).

Disorganized schizophrenia is thought to be an extreme expression of the "disorganization syndrome" that has been hypothesized to be one aspect of a three-factor model of symptoms in schizophrenia, the other factors being "reality distortion" (involving delusions and hallucinations) and "psychomotor poverty" (lack of speech, lack of spontaneous movement and various aspects of blunting of emotion).

The Fregoli delusion, or the delusion of doubles, is a rare disorder in which a person holds a delusional belief that different people are in fact a single person who changes appearance or is in disguise. The syndrome may be related to a brain lesion and is often of a paranoid nature, with the delusional person believing themselves persecuted by the person they believe is in disguise.

A person with the Fregoli delusion can also inaccurately recall places, objects, and events. This disorder can be explained by "associative nodes". The associative nodes serve as a biological link of information about other people with a particular familiar face (to the patient). This means that for any face that is similar to a recognizable face to the patient, the patient will recall that face as the person they know.

The Fregoli delusion is classed both as a monothematic delusion, since it only encompasses one delusional topic, and as a delusional misidentification syndrome (DMS), a class of delusional beliefs that involves misidentifying people, places, or objects. Like Capgras delusion, psychiatrists believe it is related to a breakdown in normal face perception.

ASPD commonly coexists with the following conditions:

When combined with alcoholism, people may show frontal function deficits on neuropsychological tests greater than those associated with each condition.

Foerster's syndrome is the name used by Arthur Koestler in his account of the compulsive punning first described by the German neurosurgeon Otfrid Foerster.

In 1929, Foerster was operating on a patient suffering from a tumor in the third ventricle – a small cavity deep down in the phylogenetically ancient regions of the midbrain, adjacent to structures intimately concerned with the arousal of emotions. When the surgeon began to manipulate the tumor, affecting those sensitive structures, the (conscious) patient burst into a manic flight of puns. He exhibited typical sound associations, and with every word of the operator broke into a flight of ideas. The sound of one word swiftly echoed in the sound of the next, and all of the words had something to do with knives and butchery. This gruesome humour, Koestler noted, all came "from a man tied face down to the operating table with his skull open".

Stendhal syndrome, Stendhal's syndrome, hyperkulturemia, or Florence syndrome is a psychosomatic disorder that causes rapid heartbeat, dizziness, fainting, confusion and even hallucinations when an individual is exposed to an experience of great personal significance, particularly viewing art. It is not listed as a recognised condition in the "Diagnostic and Statistical Manual of Mental Disorders".

Oneirophrenic patients are resistant to insulin and when injected with glucose, these patients take 30 to 50% longer to return to normal glycemia. The meaning of this finding is not known, but it has been hypothesized that it may be due to an insulin antagonist present in the blood during psychosis. However, There is currently no known treatment for oneiophrenia.

An inferiority complex is the lack of self-worth, a doubt and uncertainty about oneself, and feelings of not measuring up to standards. It is often subconscious, and is thought to drive afflicted individuals to overcompensate, resulting either in spectacular achievement or extremely asocial behavior. In modern literature, the preferred terminology is "lack of covert self-esteem". For many, it is developed through a combination of genetic personality characteristics and personal experiences.

Individuals with grapheme-color synesthesia rarely claim that their sensations are problematic or unwanted. In some cases, individuals report useful effects, such as aid in memory or spelling of difficult words.

These experiences have led to the development of technologies intended to improve the retention and memory of graphemes by individuals without synesthesia. Computers, for instance, could use "artificial synesthesia" to color words and numbers to improve usability. A somewhat related example of "computer-aided synesthesia" is using letter coloring in a web browser to prevent IDN homograph attacks. (Someone with synesthesia can sometimes distinguish between barely different looking characters in a similar way.)

Diagnosing a patient can be difficult as they are often frustrated from ineffective therapy and being told they have mental illnesses. Some patients actually have trouble deciding whether they have a taste or smell problem. In this case asking questions about food choices will help determine whether a patient has a smell or taste disorder. It is important to identify whether the distortion applies to an inhaled odorant or if an odor exists without the stimulus. The distortion of an odorant is presented in two types: the stimuli are different from what one remembers and in the second, everything has a similar smell. A clinical history can also help determine what kind of disorder one has because events such as respiratory infection and head trauma are usually indications of parosmia where as phantosmias usually have no history of such events and occur spontaneously. Unfortunately there are no accurate diagnostic tests or methods for dysosmia. Evaluation must be done through questionnaires and medical history.

Grapheme-color synaesthesia or colored grapheme synesthesia is a form of synesthesia in which an individual's perception of numerals and letters is associated with the experience of colors. Like all forms of synesthesia, Grapheme-color synesthesia is involuntary, consistent, and memorable. Grapheme-color synesthesia is one of the most common forms of synesthesia, and because of the extensive knowledge of the visual system, one of the most studied.

While it is extremely unlikely that any two synesthetes will report the same colors for all letters and numbers, studies of large numbers of synesthetes find that there are some commonalities across letters (e.g., "A" is likely to be red). Early studies argued that grapheme-color synesthesia was not due to associative learning, such as from playing with colored refrigerator magnets. However, one recent study has documented a case of synesthesia in which synesthetic associations could be traced back to colored refrigerator magnets. Despite the existence of this individual case, the majority of synesthetic associations do not seem to be driven by learning of this sort. Rather, it seems that more frequent letters are paired with more frequent colors, and some meaning-based rules, such as ‘b’ being blue, drive most synesthetic associations.

There has been a lot more research as to why and how synesthesia occurs with more recent technology and as synesthesia has become more well known. It has been found that grapheme-color synesthetes have more grey matter in their brain. There is evidence of an increased grey matter volume in the left caudal intra- parietal sulcus (IPS). There was also found to be an increased grey matter volume in the right fusiform gyrus. These results are consistent with another study on the brain functioning of grapheme-color synesthetes. Grapheme-color synesthetes tend to have an increased thickness, volume, and surface area of the fusiform gyrus. Furthermore, the area of the brain where word, letter, and color processing are located, V4a, is where the most significant difference in make-up was found. Though not certain, these differences are thought to be part of the reasoning for the presence of grapheme-color synesthesia.

Classical Adlerian psychology makes a distinction between primary and secondary inferiority feelings.

- A primary inferiority feeling is said to be rooted in the young child's original experience of weakness, helplessness and dependency. It can then be intensified by comparisons to siblings, romantic partners, and adults.

- A secondary inferiority feeling relates to an adult's experience of being unable to reach a subconscious, reassuring fictional final goal of subjective security and success to compensate for the inferiority feelings. The perceived distance from that reassuring goal would lead to a negative/depressed feeling that could then prompt the recall of the original inferiority feeling; this composite of inferiority feelings could be experienced as overwhelming. The reassuring goal invented to relieve the original, primary feeling of inferiority which actually causes the secondary feeling of inferiority is the "catch-22" of this dilemma, where the desperate attempt to obtain therapeutic reassurance and delivery from a depressing feeling of inferiority and worthlessness repeatedly fails. This vicious cycle is common in neurotic lifestyles.

Diagnosis of otodental syndrome was established using clinical, histopathological and audiometric methodologies. In normal individuals, by the age of 2-3, radiograph images should depict any signs of premolar development. A formal diagnosis of no premolar growth can be done by age 6 in order to check for signs of otodental syndrome. Sensorineural hearing loss can be another measure for proper diagnosis as well as checking for ocular coloboma. The latter is usually noticed at an around birth.

Molecular genetic testing can aid in the diagnosis of the affected individual, which would determine if there are any abnormalities in the FGF3 gene (11q13) or the FADD gene (11q13.3). Additional tests that can help diagnose otodental syndrome are ear infection tests, hearing tests, oral examination, and eye examinations to check for the specific phenotypic associations. Due to the rarity of otodental syndrome, most symptoms are looked at on an individual basis unless multiple symptoms are all apparent at once.

There is potential for differential diagnosis due to similarities in symptoms. Other diseases that share common symptoms are chondroectodermal dysplasia, achondrodysplasia, and osteopetrosis

According to Professor Emily Simonoff of the Institute of Psychiatry, Psychology and Neuroscience, "childhood hyperactivity and conduct disorder showed equally strong prediction of antisocial personality disorder (ASPD) and criminality in early and mid-adult life. Lower IQ and reading problems were most prominent in their relationships with childhood and adolescent antisocial behaviour."

Tuber cinereum hamartoma may be associated with Pallister-Hall syndrome, a diagnosis characterized by multiple malformations, including polydactyly and imperforate anus. Neurologic symptoms are less severe in Pallister-Hall than in isolated cases of hamartoma.