In visuospatial terms, a saccade is an eye movement from one area in the visual field to another, and usability testing using eye-tracking equipment tracks the timing and progression of saccades across a user interface in response to certain stimuli. These stimuli may include 1) the first presentation of the interface to the user, 2) the modification of the visual field due to animation (Flash, banners, animated GIFs, etc.), 3) the slow download of information to be presented, or any number of other things that can attract your gaze.
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