Unaccusative verb production in agrammatic aphasia: the argument structure complexity hypothesis

This study examined patterns of verb production in narrative samples of eight individuals with agrammatic aphasia and seven education- and age-matched normal subjects. Comprehension and constrained production of two types of intransitive verbs-unaccusatives whose argument structure triggers a complex syntactic derivation and unergatives that are considered syntactically simple- was also tested. Results showed that in narrative tasks a hierarchy of verb production difficulty as seen in previous studies [Aphasiology 11 (1997) 473; Brain and Language 74 (2000) 1] emerged for the aphasic participants, with a preference noted for production of verbs with a fewer number of arguments. Both normal and agrammatic subjects also showed fewer productions of unaccusative intransitive verbs in their narrative samples as compared to other verb types (supporting findings reported by Kegl [Brain and Language 50 (1995) 151]. In contrast to relatively spared comprehension of both unaccusative and unergative intransitives, the aphasic participants showed significantly greater difficulty producing unaccusatives as compared to unergatives in the constrained task. These findings suggest that deficits in accessing verbs for production are influenced by the verb's argument structure entry and led to what is referred to as the 'argument structure complexity hypothesis'. When verbs become more complex in terms of the number of associated arguments or when the argument structure entry of the verb does not directly map to its s-structure representation, production difficulty increases.