They will be tested on their ability to remember sequences of numbers, packs of playing cards and people's names.
Quietly psyching himself up was Dominic O'Brien, the five-times world memory champion. He was munching a leaf from the sacred gingko tree and shuffling a pack of playing cards, while wired up to his computer to watch the right and left sides of his brain thinking.
"Training is as meticulous as for any physical sport," says the former dyslexic, whose amazing ability to remember number sequences has had him banned from casinos. "Goethe had a gingko tree in his garden and ate a leaf a day to improve his memory. If it's good enough for him, it's good enough for me."
Mr O'Brien's bizarre training methods underline just how competitive mind sports have become. At 42, he is trying to wrest back the title after missing out last year. He announced his intention to take part only last week. "I wanted to keep them guessing, like a football manager announcing his line-up just before kick-off," he said.
The football analogy is apt: serious contestants in all 40 games will be at the peak of physical fitness too - Mr O'Brien, who can remember a deck of cards in 38 seconds, jogs three times a week and plays golf.
Mental gymnasts, however, claim they do not resort to mind-enhancing substances, apart from coffee, tea - and oxygen. All faculties need to be fully operational. Fish (its oils are good for the brain) and pasta (good for stamina) are essential parts of the diet.
Their phenomenal powers of recall rely on imaging - the number 10 is represented, for example, by a picture of Tony Blair, seven by James Bond and so on. Typically these images are taken on a journey round a golf course and meet on different fairways to create a memorable "story".
The games at the MSO fall into two categories. There are tests of strategy and tactics such as chess, shogi, Scrabble, draughts, bridge, Othello and go; and there are games that test mental skills such as memory, speed reading, mind mapping, creative thinking, and even pub quizzes - chaired by Magnus Magnusson. More than 5,000 contestants from 60 countries will be competing for prize money of pounds 100,000.
"Historically mind sports have suffered from being seen as both elitist and trivial," said MSO organiser Tony Buzan, inventor of the concept of mental literacy. "Ruling families such as the Aikido in Japan kept their games secret so that they could maximise their authority. In Britain chess was taught only at private schools. And people pooh-pooh crosswords for being a waste of time.
"But we need to play mental sports. They are gymnasiums for the mind. They train the brain to think tactically, strategically and creatively, and they teach us how to win and lose. There is medical evidence too that exercising our mental muscles increases longevity and keeps Alzheimer's at bay."
The screen culture and greater leisure time have helped to hasten the spread of mind sports. A masters' chess match, for example, will typically get 50 million hits on the internet. And virtual tournaments have brought the game to hundreds of thousands of new players: at any moment 15,000 people are playing chess electronically.
Workshops at the MSO will teach people new games. But novices will be hard pressed to emulate Demis Hassabis's feat last year. When the 22- year-old software genius from Finchley, north London, arrived at the MSO he didn't know how to play bridge. By the end Hassabis, who was a chess master at 12, had won a silver medal in bridge to add to his seven in other games. His ambition this year is quite simply to become the world's best all-round mind sports player ever.
Woody Allen once said that the brain was his second favourite organ. But the appeal of mind sports is growing so fast that they may soon oust their more brawny rivals. Games have never been so serious.