Bilingualism, brain health, and mental training

In America, we live in one of the few countries where most of the population is exclusively monolingual. Bilingualism and multilingualism are more the human norm. Jared Diamond writes, in The Benefits of Multilingualism (Science, 330:6002 (15 October 2010), 332-333.):

Multilingualism—the ability to understand and speak several languages—is exceptional in the United States but common elsewhere, especially in small-scale traditional societies. For instance, once while I was camped with some New Guinea Highlanders conversing simultaneously in several local languages, I asked each man to name each language in which he could converse. It turned out that everyone present spoke at least 5 languages, and the champion was a man who spoke 15. What are the cognitive effects of such multilingualism? Recent studies (1–5) show that children raised bilingually develop a specific type of cognitive benefit during infancy, and that bilingualism offers some protection against symptoms of Alzheimer's dementia in old people.

...Our minds are assaulted by varied sights, sounds, and other external sensory inputs, plus thoughts and proprioreceptive sensations (which make us aware of the relative positions of our own body parts) (see the figure). To succeed in doing anything at all, we must temporarily inhibit 99% of those inputs and attend to just 1% of them, and the appropriate choice varies with the circumstances. That selective attention involves a set of processes, termed executive function, that reside in the prefrontal cortex and develop especially over the first 5 years of life.

Multilingual people have a special challenge involving executive function. Monolinguals hearing a word need only compare it with their single stock of arbitrary phoneme (sound) and meaning rules, and when uttering a word they draw it from that single stock. ...Multilinguals participating in a multilingual conversation...switch frequently and unpredictably between their stocks of phoneme/meaning rules. As a result, multilinguals have constant unconscious practice in using the executive function system.

Such pervasive multilingualism can cause enormous cross-pollination and "covergent evolution" in languages. Bill Foley says, in More on Papuan Languages:

Somewhere close to a quarter of the total of the world's languages are spoken in the New Guinea region-about 1100 languages. ...The small size of many Papuan speaking speech communities has often led to persistent multilingualism in the language of adjoining communities, the development of trade jargons for interlanguage communication or the shifting of language allegiance to languages of more powerful or economically advantaged neighbours. In such a complex, fragmented linguistic situation, Papuan languages not unexpectedly exhibit a pattern of enormous cross-influence in all areas. All types of linguistic features, basic vocabulary, pronouns, grammatical patterns, discourse styles can be and have been borrowed from one language into another. This makes the establishment of genetic links among Papuan languages doubly difficult: with no documentation for the vast majority of them older than 50 years, it is problematic indeed to sift what is true genetically inherited material from what is borrowed from other languages, especially borrowing from genetically related contiguous languages or borrowings centuries old from now deceased languages.

Diamond cites the research of Ellen Bialystok; LanguageLog and languagehat point us to The Bilingual Advantage, a Q&A with Bialystok at the NYT:

Q. One of your most startling recent findings is that bilingualism helps forestall the symptoms of Alzheimer’s disease. How did you come to learn this?

A. We did two kinds of studies. In the first, published in 2004, we found that normally aging bilinguals had better cognitive functioning than normally aging monolinguals. Bilingual older adults performed better than monolingual older adults on executive control tasks. That was very impressive because it didn’t have to be that way. It could have turned out that everybody just lost function equally as they got older.

That evidence made us look at people who didn’t have normal cognitive function. In our next studies , we looked at the medical records of 400 Alzheimer’s patients. On average, the bilinguals showed Alzheimer’s symptoms five or six years later than those who spoke only one language. This didn’t mean that the bilinguals didn’t have Alzheimer’s. It meant that as the disease took root in their brains, they were able to continue functioning at a higher level. They could cope with the disease for longer.

Which reminds me somewhat of the Gizmodo post How to Make Yourself Smarter in 20 Days, which highlights the use of n-back mental training exercises for 20 minutes a day for 20 days to increase fluid intelligence for three months. Not that I've tried it, but Brain Workshop vaguely reminds me of electronic flashcards and spaced repetition software like Anki, for memorization. Executive control, fluid intelligence, and vocabulary memorization are all very different, but it seems like 20 minutes a day of abstract memory practice would get pretty boring, and something language-related may be more interesting.

EDIT
Brain Calisthenics for Abstract Ideas (NYT)