Tense morphology in German agrammatism: The production of regular, irregular and mixed verbs

This study investigates tense morphology in agrammatic aphasia and the predictions of two accounts on processing of regular and irregular verbs: the Dual Mechanism model, that is, for aphasic data, the Declarative/Procedural model, and the Single Mechanism approach. The production of regular, irregular and mixed verbs in the present, simple past and past participle (present perfect) was tested in German by means of a sentence completion task with a group of seven speakers with agrammatic aphasia. The results show a difference between regular verbs and irregular verbs. Mixed verbs were equally difficult as irregular verbs. A frequency effect was found for irregular verbs but not for regular and mixed verbs. A significant difference among the correctness scores for present tense and simple past forms was found. Simple past and past participle were significantly more difficult than present tense. Error types were characterized by pure infinitive responses and time reference errors. Neither of the above accounts is sufficient to explain these results. Correctness scores and error patterns for mixed verbs suggest that such minor lexical patterns can be useful in finding new evidence in the debate on morphological processing. The findings also highlight time reference as well as language specific characteristics need to be taken into consideration.