Eye Tracking Basics

What is eye tracking?
Eye tracking is a methodology that helps researchers understand visual attention. With eye tracking, we can detect:
  • where users look at a point in time
  • how long they look at something, and 
  • the path their eyes follow
Eye tracking has been applied to numerous fields including human factors, cognitive psychology, marketing, and the broad field of human–computer interaction. In user experience research, eye tracking helps researchers understand the complete user experience, even that which users cannot describe. 
How do we track a user's eyes?
 An eye tracker is a tool that allows user experience (UX) researchers to observe the position of the eye to understand where an individual is looking. 

Most modern eye trackers rely on a method called corneal reflection to detect and track the location of the eye as it moves. Corneal reflection uses a light source to illuminate the eye, which then causes  a reflection that is detected by a high-resolution camera. The image captured by the camera is then used to identify the reflection of the light source on the cornea and in the pupil. Advanced image processing algorithms are then used to establish the point of gaze related to the eye and the stimuli.
Eye trackers today can be built right into conventional desktop displays to create a natural environment for participants.
See where people looked, not where they think they looked
In trying to understand what users pay attention to, we cannot always rely on the participants to accurately tell us. Participants are terrible at self-reporting where they looked. For the most part, this is due to our eyes often moving involuntarily and the limits of our short-term memory. Guan et al. (2006)

Did they really see that?
The challenges with interpreting mapped fixations are that just because a fixation was registered does not necessarily mean that the user really saw it or that it "registered" in their brain. This is often the case with orphan fixations where the eye may have momentarily rested in a random area of the screen, but did not intentionally look at it. 

The clustering of a number of fixations in a particular region can provide more evidence that the user deliberately looked at something, and significantly increases the likelihood that the brain processed it. Eye trackers are not mind reading devices, and can only tell us what the person looked at, but not why.
This user definitely noticed the page title, main graphic, and certain sentences based on the clustering of fixations.