This game works specifically attention and learning memory (explicit and implicit) after encoding correspondence letter / symbol to decode a message.
Easy level: Learning the duplicate letter / code is the first step, and then we give a letter that must find the code or viceversa.
Average level: we must decode or encode a series of 3 letters despite the noise distracting.
Difficult level: more distracting elements as in the previous level. Work on 4-letter words.