FUNDAMENTALS OF GAME DESIGN, SECOND EDITION
Pattern Recognition Challenges
Pattern recognition challenges test the player's ability to spot visible or audible patterns or patterns of change and behavior. One of the most common instances of a pattern recognition challenge crops up in action games when a large number of identical enemies, each of which behaves in a predictable way, confronts the player. The player can try alternative strategies until he finds one effective against that enemy, then use that strategy to vanquish any number of enemies that attack using the same pattern. To make things harder and more interesting, the boss enemy at the end of a level usually has a different and more complex pattern of behavior from the smaller enemies that preceded it.
Visual clues often figure in pattern recognition challenges. In the original Doom, the player could find secret doorways by searching for areas of wall that looked slightly different from the norm. Brain Spa, a brain training game, requires the player to match pairs of similar-looking objects while under time pressure.
To make pattern recognition challenges easier, make the patterns shorter, simpler, and more obvious. To make them harder, make the patterns longer, more intricate, and more subtle.