Switching-back versus switching-out: Language context reveals a novel aging deficit in proactive bilingual language control.

The present study examined how bilinguals switch languages under conditions of varying contextual support for each language. Bilinguals were cued to name pictures in two contextually biased blocks, one biasing the dominant-language by cuing its use on 88 % of trials, another biasing the nondominant-language, and a third no-bias block (in which each language was used 50 % of the time). Experiment 1 tested 70 young Spanish-English bilinguals, while Experiment 2 compared a proficiency-matched subset of the young bilinguals to 40 older bilinguals tested on the same tasks. In the no-bias block, young but not older bilinguals exhibited reversed language dominance, replicating a previously reported aging deficit in global inhibition of the dominant language. In biased-language contexts, young, but not older, bilinguals switched-back to the biased language faster than they switched-out, and switching-back was not easier than switching-out, if anything, switching back was more costly (for young bilinguals in Experiment 1, and for older bilinguals in Experiment 2). Surprisingly, older bilinguals exhibited larger switch-back, but not switch-out, costs than younger bilinguals. To explain these results, we hypothesize that young bilinguals engage multiple forms of proactive control to switch languages, including inhibition of the dominant language, and proactive activation of a selected language, which they maintain even while temporarily switching out of it to facilitate imminent switches back. By contrast, older bilinguals rely primarily on reactive control to switch languages, and without proactive selection, bottom-up activation of the contextually supported language collapses upon switching out of it.