The first time I subjected myself to Dr Ryuta Kawashima's brain-ageing technology, I learned that I had the brain of a 51-year-old. I mean no disrespect to 51-year-olds when I say I found this alarming. I was planning on waiting a couple of decades before turning 51. All of us are in cognitive decline from early adulthood onwards. But how come I'd jumped so far along the road so quickly? I was jet-lagged, I protested. But Kawashima - who's a real person, but who appears in the Brain Training computer program as a disembodied bouncing head - just chuckled. He chuckles a lot, no doubt because he possesses the brain of a 20-year-old, which is what he says we should all be aiming for.
Brain exercise is big business. Kawashima's program is the market leader: the software, which runs on Nintendo DS video game consoles, has sold more than 5m copies in Japan, and has just been released in the UK as a book, Train Your Brain. The exercises, based on brain-scanning research, are designed to activate as many regions of your prefrontal cortex as possible, strengthening neural connections and even creating new ones. This is all just neuroscience-speak for the weird sensation - it actually feels almost like a literal stretching of the mind - you get when you attempt one of Kawashima's favourites, the Stroop test. To sample this yourself, say out loud the name of the colour in which each of the following words is printed, regardless of what the word itself is: Red Green Blue Black Black Green Red Blue Red Red Blue Green Black. Tired yet?
The psychological benefits of regular mental workouts are fairly mind-stretching in themselves. One study found that doing a single crossword each week could lower the risk of dementia in old age by 7%; those who did four crosswords a week were at 47% less risk than the once-a-week solvers. But playing bingo helps, too, as does reading novels aloud: oddly, it's not always the most difficult exercises that stimulate the brain the most. Kawashima even claims his research shows that easy mental arithmetic provokes more neural activity than hard mental arithmetic.
Almost anything seems to help, in fact, provided it ruptures the monotony of daily routines around which the brain adapts and grows passive. At Duke University, North Carolina, the neurobiologist Lawrence Katz has developed what he calls Neurobics, which involves activities as simple as just consciously using two of your senses at the same time. Listen to the rain and tap your fingers on the table top, while concentrating on both. Or jolt your brain by trying to control your computer mouse with the hand you don't usually use. You'll feel like an idiot at first, but when you stop, after a few minutes, the experience ends up feeling strangely bracing.
After a week of 15-minute sessions with Kawashima's software, I'm delighted to report, I was rejuvenated: I now have the mind of a 28-year-old. After a few more weeks of this, I plan on applying for a Young Person's Railcard.