Linguistic analysis of the speech output of schizophrenic, bipolar, and depressive patients

Using speech samples of 100 patients suffering from schizophrenia, bipolar illness and major depression, we addressed the question of the extent to which the linguistic abnormalities in the speech of these patients represent diagnosis-specific characteristics or constitute independent, syndrome-like dimensions of the illnesses. All speech samples were transcribed by a professional linguist who was blind to both identity and diagnosis of the patients. The majority of the deviant linguistic variables was found to be common to all three diagnostic groups under comparison, while only a few linguistic variables exhibited statistically significant between-group differences. On the other hand, when the respective variables were analysed as a multivariate entity, the variety of subtle between-group differences allowed us to discriminate between the diagnostic groups at an overall performance of 72.7% correctly classified patients. There was an almost complete lack of association between linguistic abnormalities and psychopathology syndromes. In particular, we found no correlation between the syndrome 'formal thought disorder' and the large variety of linguistic variables used in this investigation. In consequence, we conjecture that linguistically deviant speech characteristics represent an independent syndrome complex manifested at varying intensities across mental illnesses, and that this syndrome complex deserves greater attention, not only with respect to the principal understanding of the underlying disturbances, but also as a potential target of therapeutical intervention.