A cognitive neuroscientist, Ellen Bialystok has spent almost 40 years learning about how bilingualism sharpens the mind. Her good news: Among other benefits, the regular use of two languages appears to delay the onset of Alzheimer’s disease symptoms. Dr. Bialystok, 62, a distinguished research professor of psychology at York University in Toronto, was awarded a $100,000 Killam Prize last year for her contributions to social science. We spoke for two hours in a Washington hotel room in February and again, more recently, by telephone. An edited version of the two conversations follows.