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372 CHAPTER 13 Measuring the human

FIGURE 13.1
A web page annotated with eye-tracking data: lines indicating gaze paths link fixation
points annotated with time stamps, providing a trail for a series of interactions.
From Card, S.K., Pirolli, P., Van Der Wege, M.M., Morrison, J.B., Reeder, R.W., Schraedley, P., Boshart,
J., 2001. Information scent as a driver of Web behavior graphs: results of a protocol analysis method for
Web usability. In: Proceedings of the SIGCHI conference on Human factors in computing systems, Seattle,
Washington, United States. ACM. © ACM.

These possibilities have led to the application of eye tracking in many domains,
both as new forms of computer input and as the basis for research projects aimed at
using eye movements as a source of data for studying HCIs (Jacob and Karn, 2003;
Kumar, 2006). Eye tracking has been widely used as an assistive technology for
people with quadriplegia and others who are unable to use motor functions to oper-
ate a mouse, keyboard, or other adaptive input device (Hornof et al., 2004; Barreto
et al., 2008). The use of gaze control for pointing and selecting objects—eye gaze as
complementing (Zhai et al., 1999; Bieg, 2009) or replacing (Jacob and Karn, 2003;
Murata, 2006; Kumar et al., 2007) mice—has been suggested by many researchers,
leading to a variety of proposed designs. Others have explored taking eye tracking
one step further, using gaze as an input or control signal. One study of immersive, col-
laborative environments used eye trackers to make virtual avatars “look” where users