In the study, right-handed volunteers agreed to use their left hand to tap out on a keypad a series of six numbers shown to them on a computer screen. Participants tapped the numbers as quickly as they could for 10 seconds, followed by 10-second rest breaks, for a total of 36 training-and-rest periods. Brain waves recorded during the experiments revealed that the brain was rapidly replaying the keypad task dozens of times during breaks.

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