Eye-Tracking Technology

History of Eye Trackers

Eye tracking as a science can be traced back to at least the late 19th century. The French ophthalmologist Louis Émile Javal is widely attributed with reporting the first observation of eye movements during reading in 1879, noting that the eyes do not sweep seamlessly across a line of text but instead make a series of jerky pauses. Soon after, the psychological researcher Raymond Dodge constructed simple experiments that demonstrated that readers get no information from the eyes while they are in motion between pauses. This rather straightforward understanding that where the reader’s eyes pause while reading might reveal something about reading processes initiated what would become the expansive and productive field of eye movement research.

With the desire to research eye movements came the need for accurate eye-tracking instruments. Early eye trackers involved physical connections between the reader’s eye and a lightweight pointer that traced the movement of the eye on paper. Soon, however, somewhat less invasive methods were developed using focused light reflected off of the reader’s eye through a camera lens to moving film, recording the eye’s fixations and movements between fixations. Other approaches for tracking eye movements now exist, such as measuring changing optical electrical signals through electrodes placed around the eye. However, eye trackers used for research in reading have typically followed a light-based approach.

With the advent of computers and greater control over light amplification technology, tracking eye movements while reading has become far more accurate and less obtrusive. Currently, a widely used approach involves a video-based monitoring system. An invisible beam of infrared light is directed to the eye, illuminating part of the eye to create a reflection from the cornea, the location of which is then measured against the pupil. When the eye moves, the relationship between the corneal reflections and the pupil changes slightly, thus creating a measureable difference that can be converted to horizontal and vertical location data.

Eye-Tracking Data

The horizontal and vertical data produced by eye trackers allow researchers to understand what parts of a text a reader looked at and in what order different parts of the text were examined. When temporal data are added—that is, how long [Page 737]the reader looked at a given area—the amount of information available for analysis is remarkable.

The usefulness of examining the location and duration of eye fixations is based at least partially on the relationship between fixations and attention: There is a very strong link between where a reader fixates and what the reader is attending to, on a moment-by-moment basis. This means that by examining the location, order, and duration of those fixations, researchers can better understand the real-time comprehension processes of particular readers with particular texts and of readers in general.

While the eyes are excellent tools for transmitting textual data to the brain for use during reading, it is not the case that they can simply glance once at a page or screen—or even sweep over that page or screen—and provide information useful to typical reading situations. As alluded to earlier, the eyes pause while reading, and those pauses, termed fixations, are made for a reason. It is the fixations that allow the delivery of functional information, because the movements between fixations—which are called saccades—provide no usable information to the reader.

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