Rehabilitation of past tense verb production and non-canonical sentence production in left inferior frontal non-fluent aphasia

Background: Studies devoted to improving past tense verb production in Broca's aphasic patients are sparse. Of the existing studies few have produced generalised improvement on untreated verbs, although one study has shown improvement on untreated regular, but not irregular, verbs (Weinrich, Boser, & McCall, 1999). Within a single mechanism account of past tense verb production (McClelland & Patterson, 2002a, 2002b), irregular verbs fall into clusters that share similar transformations from their stem to past tense. No studies to our knowledge have explored whether strengthening irregular verb representations during rehabilitation can support production on untreated irregulars from the same irregular clusters. Aims: The aim of the current paper was to test the single mechanism claim that generalised improvement can be directed via irregular verb clusters in a Broca's aphasic participant (DS). We treated past tense verb production in sentences by using a mapping therapy, with the aim of maximising improved production of past tense verbs at both the single word and sentence level. Methods & Procedures: The study used a thematic mapping procedure requiring DS first to describe what was happening in a picture with two characters twice, beginning with different characters each time. Two thematic questions followed and DS was asked to produce the canonical and non-canonical sentences again. Non-canonical sentences required past tense verb production. Pre- and post-treatment baselines were administered to test gains in sentence production, sentence comprehension, and verb production. Generalised verb production was assessed using three untreated sets: regular verbs, irregular verbs from the same "clusters" as treated items, and irregulars from other verb clusters. Outcomes & Results: Generalised improvements were seen in the production of non-canonical sentences. Regular and irregular past tense verb production also improved, in both single word and sentence production modalities. There was significant generalised improvement to the untreated regular verb set. Interestingly, the improvement on treated irregular verbs generalised only to untreated irregulars from the same clusters as treated verbs. Conclusions: The findings suggest that past tense verb production can be improved by promoting mapping between semantic and syntactic levels of sentence processing. The generalisation to untreated verbs with similar transformations from their stem to their past tense is promising from a clinical standpoint, suggesting that generalisation may be directed in this way. Our findings also demonstrate how rehabilitation studies can contribute to the current theoretical debate between competing models of past tense verb production. Here, the findings support an account positing generalisation across clusters of irregular verbs.