Unusual language use is a core feature of psychosis, but the nature and significance of this are not understood. In particular, thought disorder in schizophrenia is characterized by markedly bizarre speech, but the cognitive components that contribute to this and the brain correlates of these components are unknown.
A number of studies have demonstrated language abnormalities in single-word processing, but few have examined speech in schizophrenia at the discourse level. This has been at least partly due to the difficulty in quantifying content of discourse. Recently, methods in computational linguistics have been found to be useful for detecting differences in semantic coherence during discourse between different clinical groups. We build on this work by demonstrating how these methods can be combined with fMRI in order to tease apart factors that underlie free discourse and its deviations, and how they relate to brain activity.
Eleven volunteers with schizophrenia and eleven controls participated in an interview during which they were asked to talk as much as they could about ‘religious belief’. These same participants underwent fMRI during a word monitoring task, during which modality of monitoring was manipulated by varying the congruence of auditory and visual stimuli. Semantic coherence scores, measured from free discourse, were examined for their relationship to brain activations during fMRI.
In healthy controls, regions associated with executive function were related to coherence. In persons with schizophrenia, coherence was mainly related to auditory and visual regions, depending on the modality of monitoring, but superior/middle temporal cortex related to coherence regardless of task. These findings are consistent with existing evidence for a role of superior temporal cortex in thought disorder, and demonstrate that computational measures of semantic content capture objective measures of coherence in speech that can be usefully related to underlying neurophysiological processes.