The n-back task is a cognitive test in which a player is presented with a sequence of stimuli (such as squares flashing in different positions on a grid or different letters being read out loud) and presses a button when the current stimulus matches the one from n steps earlier in the sequence.

Some claim that this game increases the player's working memory capacity over time with practice, which might be a proxy for fluid intelligence itself, but the evidence for both claims is controversial. If true, this would mean one could increase their IQ by regularly practicing the game.

The number of previous steps a player must remember, n, can be adjusted to make the game more or less difficult. The number of different kinds of stimuli can also be adjusted. For example, in the most popular variant of the game, "dual n-back", players are presented with two simultaneous stimuli. At each step, players see a square in one of nine positions on a grid and simultaneously hear the name of a letter read out loud, and they must press one of two buttons independently to indicate when a letter or position was repeated from n steps back.

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Gwern Branwen: Dual n-Back FAQ
Wikipedia: n-back