Ryuta Kawashima is used to children pointing at him on the street. They often shout “Kawashima Devil!” It’s the price the neuroscientist pays for being the famous face of a lucrative series of brain puzzles that he developed for Nintendo.

Kawashima appears in the videos as a disembodied, floating head with horns and a bright red face, asking “devilishly” hard maths and memory questions. Millions of games have been sold, earning him royalties of over $30m. But, he says, his games are more than just a fun way to learn: they could, in fact, provide a revolutionary new way to treat dementia.

The 54-year-old refused to keep the money he made from the brain puzzle series, ploughing much of it into a research centre in Japan’s Tohoku University, attached to the Institute of Development, Aging and Cancer. Kawashima’s 40-strong team of young scientists spends their days working on ways to train our working memory and stimulate the prefrontal cortex, the area of the brain that deals with problem-solving and personality. Brain exercises have been shown to expand the cortex of healthy young people, he says, “So why not the old?”

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That question animates a remarkable new documentary on Kawashima’s work. In Do You Know What My Name Is? pensioners with severe dementia at a care home in the US state of Cleveland, Ohio, are seen recovering the use of their memories after using a six-month programme of learning therapy he designed. Some are almost literally brought back to life, transformed from depressed, hollow shells slipping inexorably toward death back into sociable, happy people.

“We neuroscientists knew that brain plasticity exists in young subjects. The new point is that we now know it exists even in the brains of dementia sufferers,” Kawashima explains.

He says stimulating the frontal cortex clearly improves memory and brainpower: “We found that the best candidate for training working memory in people with dementia is reading aloud and performing simple arithmetic.”

Kawashima claims his own tests show an improvement in up to six out of 10 dementia sufferers, and he thinks that this can be bettered.

Dementia, a catch-all term for symptoms that include loss of memory and cognitive function, afflicts about 800,0000 people in the UK, according to the Alzheimer’s Society. The symptoms are progressive, robbing victims of memory, confidence, personality and, eventually, life as they slowly fade away. The condition costs the UK economy an estimated £23bn a year, says the Society, and there is no known cure or preventative – only the use of expensive drugs to delay the onset.

With the number of sufferers expected to treble worldwide from 36 million to 115 million by 2050, according to the World Health Organisation, scientists are increasingly turning to non-drug treatment.

A paper this month in the Proceedings of the National Academy of Sciences says a simple, cheap prescription of vitamins B6 and B12, and folic acid can slow the decline of grey matter. Nurses at care homes around the world use approaches such as music, art therapy and card games in an attempt to keep older brains stimulated and alive.

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Japan is a test case for dementia because it is the world’s fastest-ageing society, with nearly a quarter of the population aged 65 or older. For about a decade, thousands of private nursing homes across the country have been using Kawashima’s learning therapy, essentially a series of 30-minute brain exercises done every day, five days a week. The method has spread through word of mouth, after nurses and strapped local governments almost universally reported a slowing or even reversal of cognitive decline in subjects.

So far, however, Japan’s powerful Ministry of Health has so far refused to fund large-scale clinical trials, Kawashima laments. He believes he knows why: “Many doctors are not happy with our results because if they use our method they can’t sell drugs. This is a very big market in Japan and they’re losing a lot of money.”

Because his approach is marketed via a private company called Kumon Institute of Education, he insists he doesn’t, in any case, need government help. The Ministry of Health declined to comment on his method.

As Kawashima’s fame has grown, so has interest in his work – along with criticism. Some scientists have branded him a charlatan who has made a lot of money selling crank theories. Others say the jury is still out.

“I think he is a compelling character, and has some excellent ideas,” says Professor Clive Ballard, a researcher on dementia with University College London. “They aren’t yet well supported though by research evidence,” he adds.

The problem, Kawashima admits, is the dearth of large-scale clinical trials on his work. However, that may be changing soon. The Cleveland experiment is expected to spread elsewhere in the US and further afield too: Finland’s government is launching a bid to translate his learning therapy into the local language and culture.

“His work has a promise big enough to invest in a really big trial,” says Dr Juha Teperi, who is organising what he says will be a randomized sample of between 200 and 300 people. If Kawashima’s findings are repeated, “it would be a really big breakthrough,” Teperi believes.

The father of four boys, Kawashima is critical of modern lifestyles which, he says, may worsen dementia in years to come. Ironically, he is highly critical video games. Playing for too long can inhibit the prefrontal cortex, he explains, because it is a passive activity, like watching TV.

“I only allowed my sons to play video games on Saturday and Sunday, for one hour each,” he says.

Do You Know What My Name Is? recently won the Audience Favourite award at the 2013 American Documentary Film Festival. The movie uses photos and family testimony to show the mental decline of accomplished, vibrant people until they eventually forget even the names of their heartbroken children.

A care worker repeatedly introduces himself to his elderly charges with the documentary’s eponymous question. By the end of the film, remarkably, some patients can recall this information.

“It’s an example of how we can make the lives of older people liveable again,” says Kawashima.