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 I 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.