Top-down processing is the use of preexisting knowledge, expectations, and context to interpret sensory information. Perception is guided by what the mind already knows as well as by the data arriving from the senses.

Reading handwritten scrawl becomes possible because expectations about words and sentences constrain the interpretation of ambiguous shapes. Listening to speech in noise relies on linguistic knowledge to fill in missing sounds. Perceptual set and context effects are everyday demonstrations of top-down influence. The process is rapid and usually automatic; only when expectations are violated does the contribution of top-down processing become noticeable.

Top-down and bottom-up processing operate together. Purely data-driven perception would be slow and error-prone; purely knowledge-driven perception would be untethered from the world. The balance between the two shifts with the clarity of the sensory input and the strength of prior expectations. Expertise increases the efficiency of top-down guidance within a domain.

What one expects to see or hear shapes what one does see or hear.