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The rudimentary navigational tools that I have described are based on a mechanism that allows an animal to drift up or down a gradient of light, heat, magnetism, or the concentration of some chemical. Such mechanisms can serve a variety of functions where animals need to get from where they are to an easily defined target such as a strong source of light or a warm pool of water. Simple
as they are, some things are still not well understood about these elementary mechanisms. Indeed, some of the fine details of bacterial navigation have led researchers to suggest that these tiny beings possess a type of cognition not different in kind from that found in much larger multicellular animals.

When a hungry urban primate tries to zero in on Sarah’s Spicy Potatoes in the buffet line, this is yet another form of taxis, but for reasons that will soon be clear, the technical hurdles that must be overcome to reach such targets are considerably more complicated than those faced by the average amoeba or slime mold.

A frog sits motionless at the edge of a muddy stream, seemingly oblivious to the passage of time and the flow of events. When a fly happens within striking range, the frog’s tongue lashes out to capture it with such speed and precision that the fly seems to have vanished into thin air. Clever scientific experiments using time-lapse photography have shown that the frog can not only discern the direction of the fly’s movement but also assess the fly’s distance with enough precision to ensure accurate contact between sticky tongue tip and hapless fly torso.
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Though prey catching in frogs might seem very different from taxic behavior in bacteria, what they share is that they are simple behaviors designed to help an animal make a connection with a spatial target. One advantage that an animal like a frog has over a microscopic one-celled critter is simply that of size. With a big enough body, sensors can be placed in such a way that they can be used to triangulate on the location of a target. A pair of sensors—the eyes in this case—can make precise estimates of the locations of target objects without having to engage in the complicated trial-and-error methods used by much smaller animals.

Bilateral symmetry (that is, a body composed of two more or less identical halves) is common in nature, and with such symmetry comes paired sense organs. The mechanism by which pairs of sensors can produce useful orienting behaviors can be exceedingly simple. A basement hobbyist can easily construct a small machine capable of such seeking behaviors using nothing more than a pair of sensors (for example, simple light detectors that can be purchased for a few pennies at an electronics shop), a pair of wheels, and a powered motor. By wiring the machine together in such a way that each sensor is attached to a wheel on the opposite side of the body, the machine can be made to roll rapidly toward sources of light. Alternatively, reversing the wiring will produce a timid machine that seeks out dark corners.
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More sophisticated uses of paired sensors involve comparing the images that are presented to each sensor to arrive at an estimate of the location and distance of a target. When we look at an object, its image falls in slightly different locations in each of our two eyes, and our brain can compute the distance of the object based on such differences. When we listen to a sound, the differences in the qualities of sounds arriving at our two ears can be used in similar fashion to compute the location of the sound source. The power of two in this case means that animals possessing paired sensors do not need to engage in hit-and-miss games of blind man’s bluff in order to get close to the things they need. Instead, comparing the messages conveyed by each of the two sensors provides a rapid and accurate estimate of the location of a target. In simple machines built with light detectors and wheels, or in frogs and toads sitting stoically waiting for dinner to come within tongue’s reach, the use of paired sensors is a considerable advance over the simple taxic mechanisms of bacteria. In more sophisticated animals like us, many more layers of neural machinery are involved in regulating our movements
with respect to targets of interest. As preponderantly visual beasts, the story begins with our eyes.

Spend a minute or two observing how your own eye movements contribute to your perception of the world. Find a point somewhere in the room and try hard to maintain your gaze on that location. While doing this, notice how much you can see of objects just outside your fixation point. If you hold your gaze steady, you’ll notice that your perception of the rest of your setting consists of nothing more than a few blobs of varying brightness. Notice how little can been seen clearly when the eyes are held in a stationary position. Visual details are available in a small region of space around your fixation point, but nowhere else. To build an integrated view of the layout of the space we occupy, we need to move our eyes ceaselessly.

Working in the 1960s, when the technology for studying eye movements was primitive compared to the tools that are available to us today, Alfred Yarbus, a pioneer in the scientific study of eye movements, had participants in his experiments wear small mirrors that were attached to their eyeballs by means of small suction cups. (Yes, it was unpleasant. And, yes, Yarbus participated in his own experiments.)
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In some experiments, participants were asked to examine paintings while Yarbus recorded the patterns of their eye movements. When the eye-movement recordings were superimposed on the paintings so that it was possible to see what the participants had been looking at, Yarbus discovered that eye movements were not scattered randomly across the paintings; nor did they seem to carry out any kind of systematic search (such as from top to bottom or from left to right, as one might imagine a machine would do). Instead, the eyes tended to seek out the parts of the picture that were most salient. For example, an inordinate amount of
attention was paid to the eyes of the human figures in a painting. Yarbus was able to show that the pattern of eye movements seen during the viewing of a painting depended on the context of the viewing. If he asked people to answer questions about what they were seeing, their eye movements would reflect the strategies that they were using to search for answers. Our eye movements are not driven by what is biggest, brightest, or flashiest in a visual scene. They reflect the purpose of our looking.

Though Yarbus’s clever experiments stimulated legions of future researchers to use measurements of eye movements as a kind of window into our minds, he was limited by the crude technology of his day. Participants 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 are
movements 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.

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 with
exquisite 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 of the torso (think of bending over and picking up an object from the ground). In order to calculate the appropriate muscle contractions, it is crucial that our brains keep careful track of the relative positions of different parts of our own bodies as well as the appearance of the visual scene in front of us. We can do some of this work by using a specialized set of sensory receptors embedded in our joints and muscles. The outputs of these so-called proprioceptors report to our brain on the position of our body. In addition, whenever our brain sends a command to our muscles to move, a copy of that command is kept at hand in a neural filing cabinet so that we can use it to keep track of the expected consequences of each movement that we make. Our brain tries to save time by predicting the consequences of a movement before it has even taken place.

When we move our eyes, our hands, and our arms, we need only keep careful track of the relative movements of our body parts—eye relative to head, head relative to body, hand relative to shoulder, and so on. Walking changes everything. With each step,
we take flight from the surface of the planet, and when we alight we are in a new location. It is no longer sufficient to measure our own muscular contractions or motor commands to determine our exact position in space. We need an entirely new set of tools.

Carrying a full glass of beer across a crowded barroom can be tricky business. When standing still, or walking at a smooth, unchanging speed, the beer sits securely in the glass, no tidal waves of liquid threatening the floor or our clothes. But each change of direction or speed can cause the precious liquid to slosh around in the glass. Now imagine an observant and scientifically minded drinker wandering across the floor with glass in hand. She might notice that the way the beer moves in the glass is related in a very orderly way to the movements of the glass. Sudden changes of motion cause predictable reactions in the shape of the surface of the ale. In fact, a careful observer could calculate the path of the glass through space by doing nothing other than measuring and recording such changes (though she might not be the most fun person to drink beer with). To calculate accurately, she would need to note each and every movement of the surface of the fluid. If she was distracted for even a moment, or if her memory failed her, the missing data would cause her to lose track of her position completely.

Many animals, including human beings, have a specialized set of organs that sense movement in exactly the same manner as our observant beer drinker. These structures, called the vestibular system, consist of a series of interconnected chambers and tubes within the middle ear. These wondrously shaped vestibules, looking a bit like a curvy architectural creation by Frank Gehry, are filled with a viscous fluid. Inside each of these tubes is a small chunk of gelatin, studded with tiny crystals of limestone to give it added weight. As our head accelerates and decelerates through space, the blobs of gelatin wobble around just like the beer in the glass. Tiny hairs
embedded in the blobs are bent by each wobble, and these bending movements send signals to our brain.