Agrammatic production of subject-verb agreement: The effect of conceptual number

Three experiments tested the hypothesis that the deficit underlying agramma tic sentence production difficulties can be characterized as a limitation o f computational resources and that these resources are not restricted to sy ntactic processing. This hypothesis was tested by eliciting subject-verb ag reement errors in a sentence fragment completion paradigm. Sentence fragmen ts were complex noun phrases, containing a subject (head) noun and a modify ing prepositional phrase, containing a "local" noun. We varied the number o f "tokens" a singular head noun referred to. Therefore, in one condition, g rammatical and conceptual number of the head noun mismatched, whereas these numbers were the same in another condition. In Experiments 1 and 2, we obs erved an effect of this variable (i.e., more agreement errors when conceptu al number was plural and grammatical number singular) in normal controls. B roca's aphasics, on the other hand, showed no effect. Experiment 3 consiste d of a sentence/picture matching test. This test showed that the lack of ef fect with Broca's aphasics cannot be attributed to a comprehension deficit. We argue that these results are incompatible with the notion of a limitati on in resources specific for syntactic processing. Instead, we interpret th is as the result of a trade off: Broca's aphasics lack computational resour ces to take into account both grammatical and conceptual information in mor phosyntactic processing and rely on grammatical information only. (C) 1999 Academic Press.