of relying on infrared light, these low-cost solutions use off-the-shelf webcams to extract and track eye features on the face. Webcam eye tracking is most often employed in remote testing, during which participants use their computers at home or at work without having to come to a lab.
One of the current constraints of webcam eye tracking is poorer accuracy as compared to the standard infrared devices. The accuracy decreases even further when participants move around or move their computer—something that’s difficult to control in a remote session (see Figure 1.4). In addition, the rate at which the gaze location is sampled by webcams is relatively low, which greatly limits data analysis.
FIGURE 1.4 Some of the challenges of remote research with webcam eye tracking stem from the researcher’s inability to control the test environment.
Why Do the Eyes Move?
If you take a step back just for a bit, you’ll realize that when people talk about eye trackers recording eye movements, they usually take it for granted that the eyes move. Out of the hundreds of conversations I’ve had with people new (and not so new) to eye tracking, not once has anyone (not even John) questioned why the eyes move. They just do, right?
Human eyes, without rotating, cover a visual field of about 180 degrees horizontally (90 degrees to the left and 90 degrees to the right) and 90 degrees vertically (see Figure 1.5). Any time your eyes are open, the image of what you see is projected onto the retina. The retinal cells convert that image into signals, which are then transmitted to the brain. The cells responsible for high visual acuity are clustered in the center of the retina, which is called the fovea (refer to Figure 1.1). When you are looking at something directly, its image falls upon your fovea, and thus it is much sharper and more colorful than images that fall outside of the fovea.
FIGURE 1.5 When looking straight ahead, humans have a visual field of about 180 degrees but only 2 degrees of it belongs to sharp, foveal vision.
The foveal area is quite small—it spans only two degrees, which is often compared to the size of a thumbnail at arm’s length. Even though you typically do not realize it, the image becomes blurry right outside of the fovea in the area called the parafovea (2–5 degrees) and even more blurry in the periphery (see Figure 1.6). Therefore, eye movements are necessary to bring things into focus. This is an important, information-filtering mechanism—if everything were in focus all at once, your brain would be overloaded with information!
FIGURE 1.6 Top: The area that is in focus represents your foveal vision; the farther away from the fovea, the less detailed the image is. Bottom: Eye movements allow you to focus on multiple areas, giving the impression that you can see everything clearly.
How Do the Eyes Move?
Your eyes jump from place to place a few times per second (three to four times, on average). These rapid movements, called saccades, are the fastest movements produced by an external part of the human body. To prevent blurring, your vision is mostly suppressed during saccades. Visual information is only extracted during fixations, which is when the eyes are relatively motionless and are focusing on something (see Figure 1.7). Fixations tend to last between one-tenth and one-half of a second, after which the eye moves (via a saccade) to the next part of the visual field. Although there are a few other types of eye movements, saccadic eye movements, consisting of saccades and fixations, are most common and of the greatest interest to UX research.
FIGURE 1.7 Gaze plot representing eye movements of a person looking up a train schedule in a timetable. Fixations are represented as dots and saccades are shown as lines connecting the dots. The size of the dot is proportional to the fixation duration.
Why Should You Care Where People Look?
A great deal of research has established that where you place your gaze is typically associated with what you pay attention to and think about,3 especially when looking at something with a goal in mind. This is called the eye-mind hypothesis.
Yet, there are skeptics out there who do not think that knowing where people look can be meaningful in any way. The argument is usually, “I don’t have to look at something in order to see it,” which tends to be followed by, “I’m looking at your face right now, but I can still see the color of your sweater” or something of that nature.
You certainly could direct your attention to the periphery of your visual field. But if you wanted to see what color sweater someone was wearing, you would look directly at it for two reasons: (1) you can see things much more clearly when looking directly at them; and (2) paying attention to something and trying not to look directly at it is unnatural and requires conscious effort. Humans prefer moving their eyes when shifting visual attention, focusing on what they are trying to see. However, when people do not look at something directly, you cannot say for sure that they did not see it. Eye tracking only captures foveal vision, yielding no information about what was noticed peripherally. This is one of the limitations of eye tracking.
Another argument against eye tracking might be this: “People can look at something but not necessarily ‘see’ it.” Yes, that can happen. Close your eyes after you have been talking to someone face to face for a while and ask that person what color your eyes are. Many people will not know, although they have been looking at you (and presumably glancing at your eyes) for a while, and maybe even have known you for years. This is just one example of how you can look at an object but not necessarily register everything about it. Sometimes, you can even miss the entire object itself.
To sum up this discussion, a lack of fixation does not always mean a lack of attention, and fixation does not always indicate attention, but fixation and attention coincide a whole lot. Attention is actually slightly ahead of the eyes because it plans their next destination. Once the eyes move there, attention helps allocate the processing resources to what is being fixated upon. Knowing where users’ attention is directed helps the researcher evaluate and improve products, which is the focus of Chapter 2, “To Track or Not to Track.”
Why Do People Look at What They Look At?
Your visual behavior is influenced by anything that makes you look (bottom-up attention), as well as your voluntary intent to look at something (top-down attention). Bottom-up attention is stimulusdriven. Attention is involuntarily shifted to objects that contrast with their surroundings in some way. For example, bright colors and movement can make you look at something. Things that are new and
