Some of the world's top chess players went eye-to-eye in China on Wednesday in the year's highest-level "blindfold" chess tournament -- seen by some as the toughest challenge in the game.
Unable to physically see their own or their opponent's past moves, the players summoned headache-inducing levels of concentration to fight for gold in a silent conference room, lined up in front of laptop screens showing a blank board.
As each picked out a move, it was briefly displayed to their opponent opposite before disappearing -- although spectators outside were allowed a full computer representation of the match.
World number two Levon Aronian of Armenia took the men's title, while China's 18-year-old former women's world champion Hou Yifan clinched the women's gold.
"It's the ultimate pinnacle of the human brain," Beijing Sportaccord World Mind Games organiser Geoffrey Borg told AFP, adding that the blindfold tournament was "probably the world's most difficult type of chess".
Winners would even be tested for banned substances, organisers said, but declined to go into details of the list.
Contestants complained of aching heads after hours of intense concentration, having to visualise the entire state of the chess board while planning as many as 15 moves in advance.
"After playing for five or six hours, you can get terrible headaches," said Hungarian grandmaster Peter Leko, who stormed out of the competition room shaking his head after losing a match in a momentary lapse of concentration.
But Hou seemed relaxed after securing her gold, which came with a cash prize worth several thousand dollars.
"Blindfold chess is more about a player's energy," said Hou, an international relations student at a nearby university. "It makes chess more interesting."