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were required to have their heads restrained for periods as long as three minutes while viewing his pictures, and the little stalks that were attached to their eyeballs were uncomfortable and distracting. Today, it is possible to measure eye movements with great accuracy using a much less invasive method. Participants in such experiments can simply wear a pair of glasses that contains miniature cameras to record the movements of their eyes. Using this method, much has been learned about how our eyes capture critical information.
    While we move about, we use a series of quick glimpses, called fixations, interleaved with rapid eye movements called saccades. The average duration of a fixation is about half a second. Though there are slight variations, all saccades take roughly the same length of time, less than one-tenth of a second, regardless of the distance the eye travels during the movement. The greater the distance, the faster the eye moves. (Indeed, saccades are the fastest movement produced by the human body.) This detail is important because it suggests that saccades are programmed before they begin. In other words, before the eye begins to move, it knows where it is going. Generally, movements that have this property, whether they aremovements of the eyes or of missiles loaded with nuclear payloads, are called ballistic movements.
    These patterns of saccades and fixations have a definable structure to them, related to the actions that they accompany. Fixations vary in length depending on what they are for (locating an object, assisting in a movement such as grasping, checking something). These extraordinary patterns of fixation and movement are one illustration of the elegant
pas de deux
between perceiver and perceived. Our senses don’t merely take in the world. In a way, we actually
make
the world we live in through these kinds of interactions. In the most superficial way, our movements through space may resemble those of bacteria and slime molds, but our progress toward the buffet table is underpinned by an elegant and beautiful perceptual dance that is largely beyond the reach of consciousness. With great concentration, as in the exercise I encouraged you to try earlier, we can become aware of the occasional eye movement or head turn, but we couldn’t possibly have a genuine firsthand experience of the staccato visual sampling that underlies our stable perceptions of the visual world.
GRASPING SPACE
    Movements such as reaching, grasping, and walking have been the subject of intense scientific scrutiny. One reason for this is that the study of such movements has much to tell us about how perception and movement work together, but another, more significant reason is the tremendous importance of our ability to grasp and manipulate objects. Everyone has heard the old saw that the main reason human beings have come to dominate the planet is our possession of an opposable thumb. Though this is a dubious claim (I would put my money on our massive cerebral cortex rather than on our thumbs), there is no doubt that our ability to coordinate our eyes and our hands to interact with the world withexquisite precision is a major hallmark of what it means to be a human being. A few other animals have impressive abilities to manipulate objects (raccoons, for example), but no other animal comes close to our combination of speed, precision, and flexibility in organizing skilled movements using visual control.
    Though we reach for objects hundreds of times a day without a second thought, the problems that must be solved to complete these movements accurately are formidable. We must transform a viewed target location into a set of muscle contractions. If this seems easy, remember that the exact muscle contractions that are required will depend not just on the location of the image of the target on the retina but also on the position of the eye in the head, the head on the body, the arm on the shoulder, and perhaps even the orientation