A Brief History of Eye Tracking
Eye tracking was first used in reading research over a hundred years ago, so it is not a new technique. Huey’s ‘Psychology and Pedagogy of Reading’ (1908) describes early work developing the pyschology of reading, including research dating back to Wundt’s laboratory in Leipzig in 1879.
Huey, among others, was interested in eye movements and their contribution to the process of reading, how much information can be perceived during the fixation of the eye, word recognition processes and reading rate. Many of the basic concepts in use today about how eye movements relate to these matters were discovered by Huey and his contemporaries.
It was Emile Javal, a French oculist, who first noted that eyes don’t scan smoothly across a text. They actually make a series of jumps along a sequence of words. In between these jumps, called saccades, the eyes fixate for about a quarter of a second (180 msecs to 250 msecs). Huey conducted many experiments to understand how these factors influenced reading. A device called a Tachistoscope was devised to control how much visual information was presented to a participant and also its duration. It was used in attempts to understand the size of the ‘perceptual span’ or area of effective vision during a fixation - basically what the eye can take in while it is at rest.
What the eye can take in, and how it moves around visual material as it gathers information for the brain to process in the overall task of comprehension, is an important factor in understanding the effectiveness of today’s visual communications and how to optimize them.
Now, eye tracking is used extensively in investigating factors such a participants’ ‘point of regard’ or ‘point of interest’ and this information is used to gain insight into the viewer’s response to a wide variety of visual stimuli, for example, the quality or intensity of a participant’s interest in particular objects on view, or the useability and effectiveness of a human computer interface, or the attractiveness of a particular promotional display, or the effectiveness of advertising material etc.
How do Modern Eye Tracking Systems Work?
Today’s Eye Tracking systems measure what’s called the ‘Point-of-Regard’ by a technique called the ‘Pupil Center, Corneal Reflection’ method. The equipment consists of a matrix of LEDs emitting non-dazzling infra-red light, which is directed into the subject’s eyes. The light enters the retina and is reflected back as a bright disc. A corneal reflection is also created by the light but this appears as a relatively small bright dot.
These reflections are detected by an infra-red camera and the data captured processed by computer software, which, once calibrated with registration points, resolves the information into specific x and y coordinates that record the position of the subject’s gaze. It does this by identifying the centre of the pupil and the position of the corneal reflection in the field of view and then, using trigonometry, it’s able to calculate a precise point-of-regard position.
The data acquired during an extended eye tracking session can, for example, be overlaid on the original visual stimulus so that insights can be gained from the subject’s interaction with the material.
Just how that is done and what it all means is the subject of the next article in this series.
The image shows Fifth Dimension’s ‘Revelation’ Eye Tracking software displaying an example of a subject’s gaze trace.
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I can’t wait for the next article and I already know how it works. The fascinating thing is how rich the insight can be purely from watching people’s eyes in conjunction with their real-time gaze plot, what people don’t look at can be as insightful as what they do look at! I challenge anyone to observe eye tracking on a range of stimuli for 30 minutes and not come away impressed.