A change in a visual stimulus is introduced and the observer does not notice it.

Attention is diverted to another object or task so observers often fail to perceive an unexpected object, even if it appears at fixation.

Going beyond the information given and using prior knowledge to perceive stimuli in a certain way.

The way in which we interpret the information gathered (and processed) by the senses. We sense the presence of a stimulus, but we perceive what it is.