Structural neuroimaging correlates of semantic incoherence in schizophrenia

Abstract

Impairments in thought, language and communication in schizophrenia spectrum disorder (SSD) were conceptualized as formal thought disorder (FTD). In particular, disturbances of the thinking processes may be reflected by semantic incoherence and semantic perseveration in discourse dynamics in the speech of SSD. Recent advancements in natural language processing offer a way to evaluate these semantic impairments objectively by computing semantic similarity between consecutive language units. In this regard, investigating the neuroanatomical substrates of semantic similarity is an important next step. In this study, we collected speech samples from a total of 98 subjects, 64 patients with SSD and 34 healthy control subjects using a semi-structured free speech task. The semantic similarity analysis comprised the mean, variance, 5th percentile and 95th percentile of similarity values between consecutive words. We investigated the neural correlates of semantic similarity using voxel-based morphometry and surface-based morphometry, including cortical thickness, gyrification, sulcal depth and fractal dimension. The correlation analysis resulted in an association of a decrease in the lowest semantic similarities of neighboring words with a decrease in cortical thickness, not only in traditional language processing regions but also extending to bilateral frontal, temporal and parietal regions. The strongest associations were found with bilateral superior frontal and rostral middle frontal regions. We also found a negative association between the lowest semantic similarities and fractal dimension in the left superior frontal region. Altogether, our findings indicate the more integrative proposals that link semantic similarity and FTD with neurocognitive processes. These outcomes suggest that semantic similarity may not solely be an indicator of impairments in language production, but rather disruptions in higher cognitive processes in schizophrenia.