Using Swahili and English to test explanations of agrammatism

Background: This study is on time reference through verbs in two Swahili-English bilingual agrammatic speakers. Recent studies in several languages have shown that time reference through verb inflection, and more specifically through tense, is impaired in agrammatic speakers. Consequently, several theories have been proposed to account for this phenomenon. We explore three kinds of theories of agrammatism that are eligible to account for these data: (1) a deficit in Tense; (2) a deficit in Discourse Linking; (3) a Morphological-System deficit. Aims: The study investigated the patterns and degree of severity of time reference impairments in bilingual agrammatic speakers of Swahili and English. Production of past and future verb forms was examined in both languages to determine which of the explanations for verb inflection errors holds in bilingual agrammatic speakers. Methods Procedures: A sentence completion test was developed in two languages to elicit sentence constructions that refer to the past and the future. This test was administered to two bilingual agrammatic speakers of Swahili and English and three age- and education-matched control participants. Outcomes Results: The performance of the control participants on the two tests was perfect in both languages. Similarly, the two agrammatic speakers' performance in Swahili as well as future tense in English was at ceiling. However, both agrammatic speakers had selective deficits in the production of English past tense. Conclusions: The discrepancy between the English and Swahili data cannot be explained by a syntactic or discourse linking theory. Only a morphological deficit in terms of number of possible candidates for a particular inflectional form fits with the data.