Argument structure deficits in aphasia: New perspective on models of lexical production

Background: The study explores the ability of fluent and non-fluent aphasic patients to produce verbs using the appropriate argument structure, i.e., a feature specifying the number and type of participants in the event described by the verb. According to Chomsky's Minimalist Program (1995), the lexical entry of a verb contains information about the number of arguments and the thematic roles assigned, which is then mapped onto the sentence argument structure, while Bock and Levelt's (1994; see also Levelt, Roelofs, Meyer, 1999) model of lexical production assumes that the information regarding verb argument structure (VAS) is part of the lemma and is accessed before the retrieval of the phonological word form, i.e., the lexeme. Aim: Participants were tested to investigate their ability to map thematic information onto the corresponding syntactic argument structure. Methods Procedures: Seven aphasic patients (five suffering from non-fluent aphasia with agrammatism and two from fluent aphasia) and ten neurologically unimpaired individuals participated in the study. They were given a picture description task formulated in two conditions: in the first condition they were asked to provide a free description of the image, while in the second condition they had to complete the sentence structure provided by the examiner. Patients showing deficits in verb production were also tested for the use of prepositions within prepositional compounds and in a sentence context. Outcomes Results: Four agrammatic patients and one fluent aphasic patient scored a high rate of argument structure errors in the selection of the appropriate verb, which was employed with an incorrect argument structure. Furthermore, these patients tended to substitute rather than to omit prepositions when required to fill the gap in a sentence. Conclusions: Our results indicate an impaired access to (VAS) and/or in mapping the thematic role information onto the syntactic argument structure. This deficit, which was found in both the agrammatic and the fluent aphasic patients, conflicts with an alternative interpretation of verb production errors in terms of omission of the preposition introducing the manner adjunct. Data support the hypothesis of retrieval of the verb lexeme without prior access to the corresponding lemma, where information about argument structure is stored.