Agrammatic production: Interpretable features and selective impairment in verb inflection

This study investigates the ability of two Greek-speaking agrammatic individuals to produce subject-verb Agreement, Tense, and Aspect in light of recent hypotheses formulated to explain the patterns of performance observed in agrammatic aphasia. The role of frequency and (morphological) regularity of verbs in agrammatism is also explored. For these investigations a sentence completion task was developed. Overall, Aspect was found significantly more impaired than Tense, and Tense significantly more impaired than Agreement. No dissociation was observed between past and future Tense, or between perfective and imperfective Aspect, for any of the two agrammatic participants. No verb regularity effect was detected and only a verb frequency effect in the Tense condition was observed for one of the participants. It is suggested that the two agrammatic participants have a morphosyntactic deficit, while one of them has an additional lexical deficit. Moreover, it is argued that the increased processing demands of Tense and Aspect, which carry interpretable features, render them more vulnerable compared to Agreement, which bears an uninterpretable feature and is a local, strictly grammatical operation. The significantly worse performance of the two agrammatic participants on Aspect than on Tense is attributed to the more "subjective" nature of the former category.