Formal thought disorder

In psychiatry, thought disorder or formal thought disorder is a term used to describe a symptom of psychotic mental illness. It is also known as Knight's Move Thinking refering to the illogical way a Knight Moves in Chess.

It describes a persistent underlying disturbance to conscious thought and is classified largely by its effects on speech and writing. Affected persons may show pressure of speech (speaking incessantly and quickly), derailment or flight of ideas (switching topic mid-sentence or inappropriately), thought blocking, rhyming, punning, or 'word salad' when individual words may be intact but speech is incoherent.

Eugen Bleuler, who named schizophrenia, held that its defining characteristic was a disorder of the thinking process. It is important to note, however, that the delusions and hallucinations of psychosis could also be considered as disorders of thought, but that the term formal thought disorder applies specifically to the presumed disruption in the flow of conscious verbal thoughts that is inferred from spoken language. This is typically what is referred to when the strictly less accurate, but more commonly used, abbreviated term 'thought disorder' is used.

Subtypes in detail
Nancy Andreasen has given the following definitions.
 * Pressure of speech - An increase in the amount of spontaneous speech compared to what is considered customary.
 * Distractible speech - During mid speech, the subject is changed in response to a stimulus. e.g. "Then I left San Francisco and moved to... where did you get that tie ?"
 * Tangentiality - Replying to questions in an oblique, tangential or irrelevant manner. e.g. "What city are you from ?", "Well, that's a hard question. I'm from Iowa. I really don't know where my relatives came from, so I don't know if I'm Irish or French".
 * Derailment - Ideas slip off the track on to another which is obliquely related or unrelated. e.g. "The next day when I'd be going out you know, I took control, like uh, I put bleach on my hair in California".
 * Incoherence (word salad) - Speech that is unintelligible due to the fact that, though the individual words are real words, the manner in which they are strung together results in incoherent gibberish, e.g. the question "Why do people believe in God?" elicits a response like "Because he makes a twirl in life, my box is broken help me blue elephant. Isn't lettuce brave? I like electrons, hello."
 * Illogicality - Conclusions are reached that do not follow logically (non sequiturs or faulty inductive inferences).
 * Clanging - Sounds rather than meaningful relationships appear to govern words. e.g. "I'm not trying to make noise. I'm trying to make sense. If you can't make sense out of nonsense, well, have fun".
 * Neologisms - New word formations. e.g. "I got so angry I picked up a dish and threw it at the geshinker".
 * Word approximations - Old words used in a new and unconventional way. e.g. "His boss was a seeover".
 * Circumstantiality - Speech that is very delayed at reaching its goal. Excessive long windedness.
 * Loss of goal - Failure to show a chain of thought to a natural conclusion.
 * Perseveration - Persistent repetition of words or ideas. e.g. "I'll think I'll put on my hat, my hat, my hat, my hat, my hat, my hat, my hat, my hat..."
 * Echolalia - Echoing of other people's speech e.g. "Can we talk for a few minutes?", "Talk for a few minutes".
 * Blocking - Interruption of train of speech before completed.
 * Stilted speech - Speech excessively stilted and formal. e.g. "The attorney comported himself indecorously".
 * Self-reference - Patient repeatedly and inappropriately refers back to self. e.g. "What's the time?", "It's 7 o'clock. That's my problem".
 * Phonemic paraphasia - Mispronunciation; syllables out of sequence. e.g. "I slipped on the lice broke my arm".
 * Semantic paraphasia - Substitution of inappropriate word. e.g. "I slipped on the coat, on the ice I mean, and broke my book".

Diagnostic issues
The concept of thought disorder has been criticised as being based on circular or incoherent definitions. For example, thought disorder is inferred from disordered speech, however it is assumed that disordered speech arises because of disordered thought. Similarly the definition of 'Incoherence' (word salad) is that speech is incoherent.