Speaking more than one language protects the brain against cognitive decline and makes a person better at multi-tasking, researchers said Friday at a major U.S. science conference.

Being bilingual, or even learning a second language late in life, has been shown to slow the decline of some key brain functions, said Ellen Bialystok of York University in Toronto, at the annual meeting of the American Association for the Advancement of Science (AAAS).

A study co-authored by Bialystok found that people who spoke more than one language were diagnosed with Alzheimer's disease 4.3 years later and reported the onset of symptoms 5.1 years later than monolingual patients.

Another study, the results of which have not yet been published, used computed tomography, or CT, scans to show that bilinguals had the same level of cognitive decline as monolinguals even when the people who spoke multiple languages were at a more advanced stage of Alzheimer's, Bialystok said.

"One of the reasons bilingualism has these powerful mechanisms including protecting against early symptoms of dementia is because it's one way to keep your brain active," Bialystok told reporters at the meeting.

"Every little bit helps. The longer you've been bilingual, the more you use all your languages, the more fluent you are, all of those things contribute.

"Even if you're starting to learn a language at 40, 50, or 60, you're unlikely to become bilingual, but you are keeping your brain active. So you're contributing to cognitive reserve through very engaging and intense activity," she said.

Cognitive reserve has been defined by Yaakov Stern of Columbia University's Department of Neurology as the ability to recruit different brain networks to optimize brain performance.

"Bilingualism is a cognitively demanding condition that contributes to cognitive reserve in much the same way as do other stimulating intellectual and social activities," said the study co-authored by Bialystok and published in Neurology late last year.

Other studies have found that bilingual people are better than monolinguals at shutting out distractions and focusing on what's important, which makes them better at multi-tasking, Amy Weinberg of the University of Maryland said at the conference.

"Getting to some level of proficiency in a second language certainly makes you an expert multi-tasker," Weinberg, a professor of linguistics, told AFP.

"When you're speaking, all the languages you speak are turned on, and you have to activate a mechanism in the brain that allows you to limit interference from one language when talking in the other," she said.

"You're juggling all kinds of mental balls as a bilingual," she said.

This mental juggling act is what makes people who speak more than one language more adept at managing several tasks at once, agreed Judith Kroll, director of the center for language studies at Penn State University.

"The bilingual is somehow able to negotiate between the competition of the languages, and the speculation is that these cognitive skills come from this juggling of languages," she said.

But an ability to speak English, Chinese, Russian and Creole, for example, does not make a person more intelligent.

"Bilinguals simply acquire specific types of expertise that help them attend to critical tasks and ignore irrelevant information," said Kroll.