A perceptual set is a mental predisposition to perceive one thing and not another. Expectations, context, and prior experience bias the interpretation of sensory input.

Once a particular organization or identity is expected, ambiguous stimuli are resolved in line with that expectation. A blurred shape is seen as a particular object; a spoken word is heard as the one that fits the sentence context. Cultural background, emotional state, and recent experience all contribute to the set that is active at a given moment. Perceptual set can speed recognition when the expectation is correct and can produce error when the expectation is wrong.

Top-down influence is not a flaw; it is an efficient use of knowledge. The same mechanisms that create perceptual set allow rapid interpretation of familiar scenes and language. Awareness of the bias can help in situations where neutrality of perception is important, such as eyewitness observation or scientific measurement.

What one is prepared to see often determines what one does see.