Hilary D. Duncan (Concordia University), Jim Nikelski (Lady Davis Institute for Medical Research), Randi Pilon (Lady Davis Institute for Medical Research), Victor Whitehead (Lady Davis Institute for Medical Research), Howard Chertkow (McGill University, Lady Davis Institute for Medical Research), & Natalie Phillips (Concordia University, Centre for Research in Human Development, Lady Davis Institute for Medical Research)
Cognitive reserve; Alzheimer disease; mild cognitive impairment; MRI; cortical thickness; tissue density
According to the cognitive reserve (CR) hypothesis  certain lifestyle factors (e.g., physical activity, level of education) may mediate the relationship between degree of brain pathology and clinical manifestation of damage. Recent studies hypothesize that bilingualism may contribute to CR and protect against the onset of dementia [e.g., 2]. The current study intends to contribute to the small body of CR literature examining differences between the brains of monolingual and bilingual older adults and Alzheimer disease patients (e.g., [3,4]).
MRI scans were obtained from ninety-four patients of the Memory Clinic of McGill University at the Jewish General Hospital. Of this group, 68 patients were diagnosed with MCI (34 monolingual, 34 multilingual), and 26 as AD (13 monolingual, 13 multilingual) at the time of their scan. Monolingual participants spoke only one language. Multilingualism was defined according to the criterion set out by Bialystok and colleagues  for bilingualism (majority of life regularly using at least two languages). We did not control specifically for the age at which the second language was learned. Within each diagnosis group, patients were matched across language groups on age at time of scan, education, and symptom severity (Mini Mental Status Examination).
For both cortical thickness (Ct) and voxel-based morphometry (VBM) analyses, the dependent variable (vertex-level cortical thickness, voxel-level tissue density) was regressed onto age, Language group (monolingual or multilingual) and Diagnosis group (MCI or AD). Regions of interest were defined based on relevant literature, namely 1) those associated with differences between monolinguals and multilinguals, and 2) those typically associated with AD pathology in its early stages.
Uncorrected regression analyses demonstrate that multilinguals have thicker cortex in areas related to bilingualism (the right inferior frontal gyrus, right rostral middle temporal gyrus, left medial superior frontal gyrus, left rostral inferior temporal gyrus, left inferior parietal cortex, and the right ventromedial prefrontal cortex); none of these areas showed an effect of Diagnosis group. Areas associated with language processing and comprehension (right and left supramarginal gyri, left ventral inferior temporal gyrus) and with AD-pathology (left and right rhinal sulci, left and right caudal parahippocampal gyri) showed an interaction effect, with multilingual MCI showing thicker cortex than monolingual MCI patients; however, this language group advantage was lost when comparing the AD patients. These results suggest that multilingual AD and MCI patients show evidence of cognitive reserve in brain areas related to bilingualism, but that only those earlier in the disease process (MCI patients) continue to show an advantage in areas related to the disease pathology.
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