Language deterioration in bilingual Alzheimer's disease patients: A longitudinal study

•Language performance of bilingual AD and MCI patients are compared longitudinally.•AD had similar effects on the two languages in production tasks.•Proficient bilinguals have shared lexico-semantic processing for the two languages.

In the context of bilingual research, little is known about the effects of neurodegenerative disorders (NDs) on the processing of two languages in a bilingual. In a recent cross-sectional study, we showed that Mild Cognitive Impairment (MCI) and Alzheimer's disease (AD) had similar effects on lexico-semantic processes in the two languages of highly proficient bilinguals (Costa et al., Neuropsychologia, 2012, 50, 740-53). In the present longitudinal study, we extend this finding by looking at the pattern of language deterioration over time in the same population of Catalan-Spanish bilingual patients. All the participants completed three language-processing tasks (picture naming, word translation and word comprehension), both in their dominant (L1) and non-dominant (L2) language. At one year, the final group was made up of 50 patients: 15 with MCI and 35 with AD. For AD but not MCI, the language deterioration over time was the same in both languages, as previously reported in the cross-sectional study. The results are discussed in the frame of the hypothesis of shared lexico-semantic processing in highly proficient bilinguals and the influence of executive control deficits in language production.