Dual N-Back: Memory Training

Dual N-Back is a game that trains the working memory, backed by scientific research.
Every 3 seconds a square on the grid is highlighted and an audio sound plays.
If there is a match n steps back,
you must match it by tapping the left side of screen for a visual match and right side of screen for an audio match.
For example, when n = 1, the match is right after the visual/audio stimuli that just occurred.
For n = 2, it is two steps back, i.e, the sequence of audio letters playing A B A is a match for audio and you would tap the right side of screen.