Verb morphology in speakers with agrammatic aphasia: language production of English past participles

This paper reports an investigation of the production of verb morphology in English speakers with agrammatic aphasia. Our main goal was to test four accounts of the processing of (ir-)regularity by quantifying regularity using affix type and the presence or absence of stem changes. Production accuracy of regular, mixed and two types of irregular past participles (irregular 1, irregular 2) was tested in English using a sentence completion task with a group of five speakers with agrammatic aphasia. The results showed significant effects on production accuracy of whether the verb required a stem change and of time reference frame but no effect of affix type: past participles that required stem changes (mixed and irregular 2 past participles) were more difficult to produce than past participles that did not change their stem (regular and irregular 1 past participles). Moreover, the production of present continuous forms was more accurate compared to past participle forms. These results suggest that a categorical conception of regular versus irregular is over-simplified, as accuracy was best predicted by stem change rather than by regularity. This is a finding most consistent with the Stem-based Assembly model. The results of this study have implications for the design and selection of stimuli in future experiments: Experimental stimuli need to be controlled for stem changes and affix type rather than assuming that irregular verbs are homogeneous.