Читать книгу Eye Tracking the User Experience - Aga Bojko - Страница 12

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.