Bilingualism and Dementia: Cognitive Reserve to Linguistic Competency.

Despite the large number of elderly bilinguals at risk for Alzheimer's disease (AD) and dementia worldwide, significant questions remain about the relationship between speaking more than one language and later cognitive decline. Bilingualism may impact on cognitive and neural reserve, time of onset of dementia symptoms and neuropathology, and linguistic competency in dementia. This review indicates increased cognitive reserve from executive (monitoring, selecting, inhibiting) control of two languages and increased neural reserve involving left frontal and related areas for language control. Many, but not all, studies indicate a delay in dementia symptom onset but worse hippocampal and mesiotemporal atrophy among bilinguals versus monolinguals with AD. In contrast, bilinguals do worse on language measures, and bilinguals with AD or dementia have difficulty maintaining and monitoring their second language. Together, these studies suggest that early-acquired and proficient bilingualism increases reserve through frontal-predominant executive control, and these executive abilities compensate for early dementia symptoms, delaying their onset but not the neuropathology of their disease. Finally, as executive control decreases further with advancing dementia, there is increasing difficulty inhibiting the dominant first language and staying in the second language. These conclusions must be interpreted with caution, given the problems inherent in this type of research; however, they do recommend more work on the pre-dementia neuroprotective effects and the dementia-related language impairments of bilingualism.