Formal thought disorder in schizophrenia: an executive or a semantic deficit?

BACKGROUND: It has been hypothesized that the schizophrenic symptom of formal thought disorder is linked to both executive and semantic dysfunction. METHOD: Intellectually preserved schizophrenic patients with (n = 15) and without (n = 16) formal thought disorder, plus matched normal controls (n = 17) were administered four executive and four semantic tests. Tests of verbal fluency and comprehension of grammar were also given. RESULTS: The patients with formal thought disorder were significantly impaired on all four executive tests compared to the patients without the symptom. They were only impaired compared to non-thought-disordered patients on 1 of 4 semantic tests, which probed semantic associations between concepts. Naming performance did not distinguish the two groups, nor did a previously used measure of semantic fluency controlling for phonological fluency. CONCLUSIONS: The results provide support for a dysexecutive hypothesis of formal thought disorder in schizophrenia, and, in line with other studies, suggest that there may be a restricted 'higher-order' semantic deficit which spares naming.