Top-down

Meaning: Top-down processing is perception driven by what the mind already holds: prior knowledge, expectations, context, and motivation. The brain uses these to interpret incoming sensory information, so an ambiguous stimulus tends to be read as the most likely or expected thing. Bottom-up processing is the contrast: it starts with the raw features detected by receptors, such as edges, color, or pitch, and builds them into a percept.

On the MCAT: A passage describes people reading or hearing something ambiguous, such as the same smudged character read as different letters depending on the surrounding word, or a missing sound in a sentence that listeners fail to notice because context fills it in. You label the process as top-down and predict that expectations bias perception most when the stimulus is ambiguous or degraded. Related ideas include schemas and perceptual set.

Common confusion: Students sometimes think bottom-up means the brain is not involved. The brain is involved in both; the labels describe the direction of influence, either from higher cognition down onto sensory interpretation, or from sensory input up to a percept. Top-down is also not the same as deliberate reasoning, since it often runs automatically. Most everyday perception combines both.