2N BacK

Dual n-back is a working-memory exercise. A square lights up somewhere on a grid while a letter is read aloud, and you flag it whenever the current square or the current letter matches the one from n steps back. At 2-back that means holding two separate streams in mind and checking each against something two trials old, while new items keep arriving every three seconds. It is hard, and it stays hard, because the level moves with you.