Grammar, and Little Else, is at the Core of Formal Thought Disorder

Most of my PhD was spent reading what other people had written about formal thought disorder (FTD). Only near the end, when I analysed my own data, did I discover what it actually showed. Many of the assumptions I had developed over seven years of reading the literature turned out to be wrong.

FTD, sometimes called thought disorder or disorganised speech, is a core symptom of schizophrenia, although it can also be observed in conditions such as mania and autism. Importantly, many of the features associated with FTD can also be found in people with no psychiatric diagnosis.

In the early twentieth century, Emil Kraepelin and Eugen Bleuler were deeply interested in the language and communication difficulties associated with schizophrenia. Both believed schizophrenia reflected an underlying biological disease process and regarded FTD as one of its defining characteristics.

Interest in the phenomenon continued throughout the twentieth century, eventually culminating in the 1970s and 1980s with Nancy Andreasen's Thought, Language and Communication (TLC) Scale. The TLC is a 16-item checklist of speech and communication behaviours that Andreasen considered indicative of thought disorder. Even today, it remains the dominant assessment tool and forms the basis of several later instruments, including the TLI, TALD, KIDDIE and CLANG.

A substantial part of my thesis examined how Andreasen developed the TLC. What surprised me was how much earlier work she discarded. Concepts described by Kraepelin, Bleuler, Cameron, Wing, Kleist and others were removed, while entirely new categories were introduced. In effect, the modern understanding of FTD appears to rest heavily on one psychiatrist's decision to substantially revise the concept. Andreasen's justification was that the discarded phenomena were not commonly observed in her clinical practice. From a methodological perspective, I found that rationale unconvincing.

At the same time, linguists had been relatively quiet on the subject despite FTD being, at least on the surface, a problem of language and communication. One of my first tasks was therefore to compare pre-TLC descriptions with those contained in the TLC and classify them according to linguistic levels.

What exactly did Kraepelin mean by "derailment"? How did that differ from Andreasen's use of the same term? The more I compared historical and modern accounts, the more I realised they were all describing variations of the same four phenomena: grammar, word selection, thought completion, and discourse tracking.

I then set out to test this framework empirically.

I interviewed fifteen people diagnosed with schizophrenia and fourteen people with no self-reported psychiatric history. After transcribing their speech and compiling the material into an open-access corpus, I examined how often these four categories of communication difficulty occurred.

The results were not what I expected.

After years of reading about neologisms, semantic paraphasias, and other supposedly distinctive features of thought disorder, I anticipated finding clear qualitative differences between the groups. Instead, I found remarkably similar patterns. People without schizophrenia also produced grammatical errors, used unusual word choices, abandoned thoughts midway through speaking, and occasionally lost track of the conversational topic.

The difference was not the type of error. It was the frequency.

Participants with schizophrenia displayed these difficulties much more often, and participants diagnosed with FTD displayed them most frequently of all. Rather than revealing a unique language disorder, the data suggested that FTD may represent an amplification of communication difficulties that occur in ordinary speech.

This finding challenged much of what I had learned from the literature. Traditional descriptions often imply that people with FTD produce speech that is fundamentally bizarre or qualitatively different from that of healthy speakers. My data did not support that view.

Instead, I saw people grappling with the same communicative challenges everyone faces, only under much greater difficulty.

This interpretation led me towards the dysexecutive hypothesis of FTD: the idea that thought disorder is primarily rooted in difficulties with executive functioning associated with the prefrontal cortex. I found only limited evidence for the dyssemantic hypothesis, which proposes that semantic processing abnormalities are the primary cause. While some semantic disturbances were present, they were neither common nor severe enough to explain the broader pattern of results.

To explore this further, I conducted a second analysis using semantic classification techniques. This allowed me to measure how far participants moved away from the conversational topic during discussion.

Here I found a different pattern. Participants with schizophrenia referred to emotionally significant and autobiographical topics between sixteen and thirty-two times more often than control participants. Neutral topics were far more likely to trigger personally relevant or emotionally charged associations.

Taken together, these findings suggested that FTD is best understood as both a dysexecutive and dyssemantic phenomenon. The communication difficulties themselves appeared to be driven primarily by executive-function problems, while the tendency to drift towards emotionally salient topics pointed to broader semantic activation.

One finding stood out above all others.

Across the entire dataset, grammatical disturbances were by far the most common form of communication difficulty. This is striking because the literature often emphasises bizarre, flamboyant, or nonsensical speech. That was not what I observed. Most participants were trying hard to communicate clearly. Their difficulties were usually much more mundane: tense agreement errors, disrupted sentence construction, and other problems associated with the mechanics of speech production.

Even years after completing the thesis, I still think about these results.

For more than a century, researchers have often described the language of schizophrenia using highly abstract and sometimes opaque terminology. Yet when we examine what people actually say, the picture appears far less dramatic than terms such as Cameron's "interpenetration of themes" might suggest.

What I found was not evidence of incomprehensible speech, but evidence of ordinary communicative difficulties occurring with extraordinary frequency.

If these findings are correct, clinicians may benefit from paying closer attention to subtle changes in everyday language use, particularly grammatical deterioration, rather than focusing exclusively on the most extreme and obvious manifestations of thought disorder.