Revisiting which language declines more in Spanish-English bilinguals with Alzheimer's disease: Longitudinal decline patterns on the multilingual naming test

Abstract

Theories of bilingual language production predict that bilinguals with Alzheimer's disease (AD) should exhibit one of two decline patterns. Either parallel decline of both languages (if decline reflects damage to semantic representations that are accessed by both languages), or asymmetrical decline, with greater decline of the nondominant language (if decline reflects reduced ability to resolve competition from the dominant language with disease progression). Only two previous studies examined decline longitudinally with one showing parallel, and the other asymmetrical, decline. We examined decline over 2-7 years (3.9 on average) in Spanish-English bilinguals (N = 23). Logistic regression revealed a parallel decline pattern at one year from baseline, but an asymmetrical decline pattern over the longer decline period, with greater decline of the nondominant language (when calculating predicted probabilities of a correct response). The asymmetrical decline pattern was significantly greater for the nondominant language only when including item-difficulty in the model. Exploratory analyses across dominance groups looking at proportional decline relative to initial naming accuracy further suggested that decline of the nondominant language may be more precipitous if that language was acquired later in life, but the critical interaction needed to support this possibility was not statistically significant in a logistic regression analysis. These results suggest that accessibility of the nondominant language may initially be more resilient in early versus more advanced AD, and that AD affects shared semantic representations before executive control declines to a point where the ability to name pictures in single-language testing block is disrupted. Additional work is needed to determine if asymmetrical decline patterns are magnified by late age of acquisition of the nondominant language, and if more subtle impairments to executive control underlie impairments to language switching that occur in the earliest stages of AD (even preclinically).

Keywords: Alzheimer's disease; Bilingualism; Language decline; Language dominance; Picture naming.

Fig. 1.

Probability of correct response on MINT scores as a function of language (dominant, nondominant), time point (first, last testing session), and item difficulty across easy (top) and hard items (bottom). Dots represent actual data and lines represent predicted data.

Fig. 2.

Proportion decline by dominance group at the last testing session relative to the first testing session. Note. The x-axis shows English age of regular use in ascending order (i.e. earlier ages of acquisition are on the left and later ages of acquisition are on the right). Additionally, corresponding MINT scores of each individual participant in the nondominant language at the first session is added below. Negative values in percent decline indicate an increase in scores between first and last session. In Spanish-dominant bilinguals, all but one participant showed greater decline of the nondominant than the dominant language. In English-dominant bilinguals decline patterns varied more between participants (e.g., four showed parallel decline, one showed greater decline of the dominant than the nondominant language). Note. MDP = missing data point. Arbitrarily positioned at the far-right of the x-axis (age of regular use for this person was not available).