The formation of contextually dependent expectancies is an important feature of music cognition. Both explicit and implicit knowledge about the structure of a piece of music serve to establish highly specific expectations about the pitch, timbre, and other features of ensuing musical information. Musical expectancies represent a specific type of musical imagery. On the one hand, musical imagery might be thought of as a mental process that occurs over an extended period as a person imagines hearing or performing a piece of music. This type of imagery differs from expectancy formation in that it may transpire in the absence of sensory input. Active expectancy formation, on the other hand, generally requires that specific images for subsequent sensory input are based on preceding sensory input and established knowledge of what type of sensory input to expect.