Morpho-syntactic and morpho-phonological deficits in the production of regularly and irregularly inflected verbs

Background: The background to the study is the debate in relation to the English regular/irregular past tense forms. Aims: The purpose of the investigation was the evaluation of the dual mechanism (DMT: Pinker, 1999; Marslen-Wilson & Tyler, 1997, 1998; Ullman, Corkin, Coppola, Hickok, Growdon, Koroshetz, et al., 1997) and connectionist single mechanism models (SMT: Bird, Lambon Ralph, Seidenberg, McClelland, & Patterson, 2003; Joanisse & Seidenberg, 1999; Patterson, Lambon Ralph, Hodges, & McClelland, 2001) through exploring the reading and oral production of regular and irregular past tense forms and other verbal and nominal inflections by a Broca's type aphasic and phonological dyslexic patient. Methods & Procedures: Eight experimental tasks are reported. Three involved the reading of stems and inflected verbs and nouns in differently organised lists, two involved the oral production of past tense verbs and plural nouns, and three explored the ability to distinguish between written verbs inflected with various affixes. Outcomes & Results: In reading randomly organised list of nouns, verb stems, and regular and irregular past tense forms the patient displayed dissociation between regular and irregular past tense forms as predicted by DMT. When the same items were presented in a list with present and past tense forms paired, and in the oral transformation task, the dissociation disappeared, and performance in regular and irregular past tense forms became comparable. There was a difference in the patient's reading of plural nouns and progressive verbs, which was good, and of past tense forms and third person forms, which was impaired. The recognition/comprehension tasks revealed that the patient was aware of the presence of an affix, but he could not reliably distinguish between different affixes. Conclusions: Performance on regular/irregular past tense forms and the variable levels of performance in producing different regular inflections are in conflict with both DMT and SMT on a number of grounds. The task-related differences between randomly organised lists and paired present and past tense forms are accounted for by distinguishing between morpho-phonological and morpho-syntactic effects. It is argued that deficits confined to the production of regular past tense forms are morpho-phonological in nature, while deficits in both regular and irregular past tense forms originate in morpho-syntax. Since SMT and DMT are theories of morpho-phonological processes, they cannot account for the complex performance pattern presented by the patient in the present study and by other similar patients. The differences attested in the availability of differently affixed words and deficits in irregular past tense forms are only accountable at the level of morpho-syntax.