Eye Movements and Action in Everyday Life

Eye Tracking in Natural Behavior

Despite this importance, the study of eye movements and their role in visual perception and action has historically been restricted to simple controlled experimental conditions. Part of the reason for this has been the general strategy in experimental psychology of investigating the simplest components of perception and movements with the goal of understanding complex behavior in terms of its elemental components. Perhaps the more important reason, however, has been the difficulty of measuring eye movements in an unconstrained observer. The high optical power of the eye means that a tiny eye rotation translates into a significant change in the direction of gaze in the world, such as a change in gaze from one word to the next during reading. To measure such tiny rotations, early eye position measurement devices involved awkward and often painful attachments to the eye that necessitated stabilizing the head. Subsequent systems, such as electromagnetic eye coils and the dual Purkinje image tracker were more comfortable, but still required the head to be held fixed. However, during the last 10 to 15 years, the advent and rapid refinement of eyetrackers mounted on the head have allowed considerable freedom of movement. Michael Land built one of the first of these, and a recent version by Jason Babcock and Jeff Pelz, shown in Figure 1(a) (see also the color insert, Figure 24a), is completely portable, using power from a battery carried in a small backpack. The new eyetrackers allow the study of eye movements over extended tasks in natural settings. As well as providing eye position in the head, or in space, these trackers also provide a video from the subject's viewpoint, with eye position superimposed. This is particularly useful in interpreting the eye movements because it shows the stage of the task corresponding to a particular gaze point.

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