There is a period of up to six hours after an event in which it is possible to affect people's memories of it. This could prove particularly useful in cases of post-traumatic stress disorder in which sufferers have repeated flashbacks of traumatic events. Researchers at Oxford University showed a film to 40 healthy participants, which included traumatic images taken from sources such as road-safety advertisements. After waiting for 30 minutes half the people played the computer game Tetris for 10 minutes while the others did nothing. Those who played the game (beloved of office procrastinators the world over) which involves manipulating shapes composed of square blocks that fall down the screen to create a horizontal line of blocks without gaps had far fewer flashbacks to the film over the next week. The researchers thought that recognising the shapes and moving the blocks around the screen competes with, and to some extent displaces, the visions of trauma retained in the sensory part of the brain.
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