Proverb comprehension reconsidered--'theory of mind' and the pragmatic use of language in schizophrenia.

BACKGROUND: For decades, impaired proverb comprehension has been regarded as typical of schizophrenic thought disorder. Testing patients' proverb understanding has widely been abandoned, however, due to poor reliability and validity of the assessment procedures. Since the underlying cognitive deficit of impaired proverb interpretation remained obscure, this study sought to determine the relation of proverb understanding with other cognitive domains, particularly 'theory of mind' or 'mindreading', in schizophrenia.
METHODS: 31 patients diagnosed with schizophrenia were assessed using a novel German Proverb Test [Barth, A., Küfferle, B., 2001. Die Entwicklung eines Sprichworttests zur Erfassung konkretistischer Denkstörungen bei schizophrenen Patienten. Nervenarzt 72, 853-858.], a 'theory of mind' test battery, a variety of executive functioning tests and verbal intelligence. Psychopathology was measured using the PANSS [Kay, S.R., Opler, L.A., Lindenmayer, J.P., 1989. The Positive and Negative Syndrome Scale (PANSS): rationale and standardisation. Br. J. Psychiatry 158 (suppl. 7), 59-67.]. Patients' task performance was compared to a group of healthy control persons.
RESULTS: 'Theory of mind', executive functioning and intelligence were strongly correlated with patients' ability to interpret proverbs correctly. In a regression analysis 'theory of mind' performance predicted, conservatively estimated, about 39% of the variance of proverb comprehension in the patient group.
CONCLUSIONS: The ability to interpret such metaphorical speech that is typical of many proverbs crucially depends on schizophrenic patients' ability to infer mental states. Future studies may further address differences between schizophrenia subtypes or the relation to specific symptom clusters.