Eye-tracking equipment monitors a participant's eye movements across a computer screen, recording the path of their gaze and points of fixation. The outputs of an eye-tracking study include "heat maps" that use color to indicate the screen areas with the greatest number of fixations, and "gaze plots" that depict eye movements across the screen.
Some usability professionals feel that eye-tracking augments conventional usability testing by providing additional data that participants can't verbalize. Others (including us) have fundamental concerns about what heat maps and gaze plots actually tell us about a user's experience. While eye-tracking is a method we have used, we feel it has inherent limitations.
Eye-tracking studies rest on the assumption that eye fixation is equivalent to seeing � if your eyes looked at it, you must have seen it. But human perception is not that simple. Sensation and perception are related but independent processes; if we processed everything our eyes fixated on, our brains would be overloaded. Heat maps run the risk of overstating the extent to which users attend to specific areas of the interface, because they only reflect the fixation, not the perception.
While eye-tracking may be useful in limited circumstances, we believe there are many less-invasive, lower-cost alternatives for understanding what users perceive. 
