Bilingual language intrusions and other speech errors in Alzheimer's disease

Abstract

The current study investigated how Alzheimer's disease (AD) affects production of speech errors in reading-aloud. Twelve Spanish-English bilinguals with AD and 19 matched controls read-aloud 8 paragraphs in four conditions (a) English-only, (b) Spanish-only, (c) English-mixed (mostly English with 6 Spanish words), and (d) Spanish-mixed (mostly Spanish with 6 English words). Reading elicited language intrusions (e.g., saying la instead of the), and several types of within-language errors (e.g., saying their instead of the). Patients produced more intrusions (and self-corrected less often) than controls, particularly when reading non-dominant language paragraphs with switches into the dominant language. Patients also produced more within-language errors than controls, but differences between groups for these were not consistently larger with dominant versus non-dominant language targets. These results illustrate the potential utility of speech errors for diagnosis of AD, suggest a variety of linguistic and executive control impairments in AD, and reveal multiple cognitive mechanisms needed to mix languages fluently. The observed pattern of deficits, and unique sensitivity of intrusions to AD in bilinguals, suggests intact ability to select a default language with contextual support, to rapidly translate and switch languages in production of connected speech, but impaired ability to monitor language membership while regulating inhibitory control.

Keywords: Alzheimer’s disease; Bilingualism; Diagnosis; Reading-aloud; Speech errors; Switching.

Figure 1

Mean percent of words with intrusions for each condition and group. The error bars represent 95% Confidence Intervals. Switch-out points refer to words that switched out of the language most words in the paragraph were written in; switch-back refers to words that switched back to the paragraph main language immediately after the switch-out points; non-switch refers to all words that were not switch-out or switch-back words. Dominant vs. non-dominant refers to the language of the target word. Mixed-language vs. single-language refers to paragraph type; mixed-language paragraphs had language switches whereas single-language paragraphs were written entirely in one language.

Figure 2

Mean percent of words with within errors for each condition and group. The error bars represent 95% Confidence Intervals. Mixed-language vs. single-language refers to paragraph type; mixed-language paragraphs had language switches whereas single-language paragraphs were written entirely in one language.

Figure 3

Mean percent of words that elicited each error type (See Table 2). The error bars represent 95% Confidence Intervals.

Figure 4

Language dominance effects i.e., non-dominant minus dominant difference scores in % errors for most commonly produced error subtypes. Positive values indicate that bilinguals produced more errors with non-dominant than with dominant language targets, while negative values indicate more errors with dominant than non-dominant language targets. The error bars represent 95% confidence intervals. Note that the intrusion errors in this figure included those produced on switch-out words in mixed language paragraphs only, while all other difference scores were calculated collapsing together errors produced on all target words in all the paragraphs.

Figure 5

Receiver Operating Characteristic (ROC) curves comparing sensitivity and specificity for discriminating cognitively healthy bilinguals from those with probable AD. Panel (A) plots intrusion errors produced with dominant language targets. Panel (B) plots all intrusions (collapsed across target language). Panel (C) plots inflection errors produced with dominant language targets. Panel (D) plots the function word substitution errors (collapsed across language dominance). Table 4 shows detailed results of the ROC analyses.