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Holmes's idea is that playing Tetris after a shocking event would take up the same mental resources that would normally be used to consolidate future flashbacks. In doing so, the notoriously addictive game could act as a "cognitive vaccine" against PTSD and provide an ironic example of a video game actually being good for you...
Holmes recruited 40 volunteers and showed them an unpleasant 12-minute film including graphic scenes of human surgery, fatal road accidents and drowning (although not Adam Sandler - there are some things that ethics boards just won't allow). Thirty minutes later, half of the group played Tetris for ten minutes, while the other half sat quietly.
Over the next week, the recruits noted down every flashback to the traumatic video in a diary. These records revealed that the Tetris players experienced less than half as many flashbacks as the group who never touched the game. And when the volunteers were brought back to the lab, a series of 32 true-or-false questions about the video showed that both groups had remembered the same level of detail about what they saw. Their memories were all intact, but their reactions differed. The Tetris players scored lower on the Impact of Events Scale, a tool used in the clinic to measure the strength of a person's response to a traumatic experience.