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The vestibular system works remarkably well for controlling certain types of movement. For example, our ability to maintain fixation on a visual target as we walk around or even leap through the air is largely brought about by a precise dialogue between our vestibular system and our eye muscles. But as a device for keeping track of our movements in larger-scale space, the vestibular system has the same weakness as our beer carrier. Errors creep into the mix, and those errors accumulate over time. Without any help from other sources, the vestibular system will become lost and disoriented. One possible source of help comes from the visual system, which has specialized abilities to keep track of our position as we move through space.

Our understanding of how vision contributes to our perception of space and motion advanced when a newly minted researcher, James Gibson, co-opted to the U.S. Air Force during World War II, stood on a runway watching fighter planes landing.
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Without question, landing is the most difficult part of flying—seasoned pilots will tell you that the definition of a successful landing is one that you can walk away from. In wartime, when new pilots needed to be trained quickly and in large numbers, there was a tremendous incentive to understand what made landing an aircraft so difficult. There was also great interest in developing a psychological test that might predict a person’s aptitude for flying. Both of these problems fell to Gibson. Gibson must have been acutely aware of the personal consequences of failure. One of his predecessors had presented potential pilots with brief glimpses of the silhouettes of different types of aircraft and then asked them to identify the shadows. This very difficult task was an abysmal failure in predicting flying aptitude, and its inventor
was discharged to the front lines. When John Watson, who later became an important figure in twentieth-century psychology for his theories of learning, was assigned the flying aptitude task, he found a way to pass the job along to a colleague, perhaps saving himself from the embarrassment of failure.

James Gibson showed more perseverance than his predecessors and eventually came to realize that good pilots kept track of their direction of movement, their altitude, and their velocity by taking advantage of certain regular patterns of visual motion that were produced by a moving observer. Gibson called these patterns optic flow, and he considered them to be at least as important to our sense of our own position as the signals we received from our vestibular system. As we move forward, the images of different parts of the world sweep across our retinas, but the region of space whose image enlarges most slowly indicates our direction of motion and our target. What this means to a pilot is that as his aircraft arcs toward the ground, the part of the planet’s surface that appears to be expanding most slowly, called the focus of expansion, is his point of interception with the ground. One part of being a good pilot is developing an understanding of how such optic flow information can be used.

The patterns of visual motion that Gibson described guide our movements all the time. While driving a car, for example, we can gauge our direction of motion using the focus of expansion. In a similar vein, as we approach a target, we can calculate when to slow down and stop in order to avoid a collision using simple calculations based on measurements of optic flow. Our ability to avoid being struck by oncoming projectiles, such as knowing when to duck to avoid being conked by a baseball, is also based on these kinds of calculations. There is even some evidence suggesting that human beings, and many other animals, possess specialized neural
circuits for detecting and responding very rapidly to these visual motions.

There is little doubt that Gibson was correct in his surmise that we use optic flow to complete simple orientation movements similar to those that can be observed in animals looking for light, darkness, warmth, or food. All of these patterns of visual motion, both those caused by our own movements and those caused by movements of objects in the world, could theoretically be used to compute our position and so help us to know our place in the world. As we will see later, the calculations that are involved can become enormously complicated, and it isn’t at all clear that we can carry them out very accurately, especially when our movements take us on the complicated paths of travel that characterize our everyday behavior.

The simplest kinds of problems in navigation involve nothing more than finding a way to decrease the distance between oneself and a target that can be sensed directly. As we’ve seen, these kinds of problems can be solved using nothing more than some basic sensors, a means of movement, and some biological wiring that joins the two together. For a one-celled animal seeking sustenance in a lakebed, a sowbug on its way to the dark, moist underside of a rock, or even a basic robotic device, things can be just that simple. Though we humans share these basic elements with all other animals, our guidance mechanisms are embedded in a much larger and more complicated system. Our ceaselessly moving eyes perch atop a complicated tower of flesh, flicking from one viewpoint to another in an elegant dance that helps us to put together an overall view of the world. The basic rules that get us from the street corner to the bus stop, or from the kitchen table to the front door, may not differ substantially from those used by bacteria, insects, or other
simple beings, yet the detailed differences in how we use our senses to construct a sensory world will assume increasing importance as this story progresses.

Many of the everyday challenges of space may involve nothing more than finding a way to move toward a target that is clearly visible, but this is hardly what we think of as wayfinding. More challenging and interesting tasks involve seeking out targets that cannot be seen directly. Here we enter a new realm where we find positions by using the relationships among things, rather than the very simple changes in the apparent size, shape, and strength of sensory signals that characterize our use of taxic mechanisms.

O
ne of the worst jobs I ever had was poring through old life insurance records to discover the names and birthdates of children of policyholders so that the company I worked for could create a computer program to send out birthday cards to those children. Though the job was staggeringly dull, there was one saving grace. The office I worked in was near the top of a skyscraper on the outskirts of downtown Toronto, and from its south-facing windows, I was able to watch the construction of the CN Tower, until recently the tallest free-standing structure in the world. The highest parts of this tower were built using a magnificent Sikorsky Skycrane
helicopter, an undertaking of such significance that the schedule of appearances of the machine was published in some local newspapers and broadcast on the nightly news. I had a front-row seat, free of charge, provided I could master the art of pretending to fill computer coding sheets with names and dates while watching the tower take shape.

As a young man whiling away his hours at a boring job, I had no sense of the transformative effect the tower would eventually have on the city. The main justification for the structure was that the boom in high-rise construction in downtown Toronto had begun to impede various kinds of radio telecommunications. But the rationale clearly had as much to do with establishing a “world-class” landmark for the city, an identifiable icon of space-age advancement, as it had to do with the pragmatics of transmitting radio waves and microwaves. But as well as serving as a landmark in the more colloquial sense of the word—as a structure whose silhouette has become identifiable as a part of the Toronto skyline as readily as New York City’s Empire State Building or Seattle’s Space Needle—the CN Tower has come to serve as a true landmark in the navigational sense. Wherever you are in the core of the city, or even in the outer fringes, it doesn’t take much of an effort to find the tower and thereby to help fix your own location. (The positional fix is helped along by the fact that the tower is located near the north shore of Lake Ontario, so one is very unlikely to be south of the tower.) The tower can also be used to gauge one’s distance from the downtown core. When I drive into the city, along a highway that skirts the edge of Lake Ontario, the easiest way to judge my progress is to take a fix on the apparent size of the tower.

Used in this way, the CN Tower is a classic navigational landmark. Though the tower itself is most often not our final destination, we can find our goal by its relationship to the tower. We have
gone slightly beyond the realm of navigating toward targets that we can see. We are no longer following plumes of the delicious aromas arising from buffet tables, or flying airplanes on to clearly visible runways using tools not conceptually different from those employed by everything from the
E. coli
bacteria in our guts to rattlesnakes hunting down field mice. Now we are using the visible to find our way to what is invisible. To do this means to have at least an implicit understanding of the spatial relationships between things. Such abilities place us in slightly more rarefied company.

Some of the first conclusive studies demonstrating animals’ use of landmarks for navigation were conducted by the biologist Nikolaas Tinbergen. Though he was eventually awarded the Nobel Prize for his studies of animal behavior, Tinbergen was the black sheep in his family. In contrast to his industrious brother, Jan (who also won a Nobel Prize, for economics, in 1969), “Niko” was known for spending dreamy summer vacations observing and photographing animals rather than applying himself in a rigorous manner to any of the established branches of zoology.
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What was most remarkable about Tinbergen was his flair for close observation of the behavior of animals followed by elegant and convincing field experiments that highlighted the principles underlying his observations.

On one family vacation, Tinbergen spent some time observing digger wasps. These wasps dig small nests in the ground, which they provision with captured and paralyzed insects so that when their young hatch they will have a larder awaiting them. This set of behaviors requires the wasps to repeatedly leave the nest and then return to it. Tinbergen wondered how the wasps found their way back to the tiny, almost invisible entrances of their nests.

Tinbergen’s approach was characteristically simple yet effective. Wondering whether the wasps might be using visual landmarks to locate the nest entrance, he simply removed some of the natural
objects that lay scattered near the entrance, sat back, and waited to see what would happen. When the wasps returned, it was clear to him that they had become disoriented. Tinbergen’s experimental coup de grâce came from a further experiment in which he seized control of the situation by replacing the objects surrounding the nest entrance with a carefully arranged circle of pinecones. Once the wasps overcame their confusion and regained their ability to find the nest, Tinbergen shifted the ring of cones to a nearby location. When the wasps returned, they searched for the nest entrance in the center of the displaced array of cones, demonstrating that Tinbergen had correctly identified the manner in which wasps found their way home.
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This simple, informal style of experiment has come to form the backbone of studies of landmark navigation in animals ranging from the wood ant to the human being. Today, though the sophistication of the methods used has advanced to a state that Tinbergen could not have imagined, the logic of the experiments has changed very little.

Tom Collett, an experimental biologist at the University of Sussex in England, has spent a lifetime studying spatial navigation in insects.
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Collett’s taste in species has been democratic—he has worked with a variety of ants, bees, and wasps, so his findings generalize well. One question that preoccupies Collett is the way insects memorize configurations of landmarks. What do they look for as they try to return to the nest? Despite the small size of their brains, insects show surprisingly sophisticated cognition. When leaving a place to which they need to return, flying insects carry out highly structured orientation flights, in which they turn to face the goal and carry out a series of swooping arcs around it. Their purpose is to produce a kind of image or snapshot of the vista surrounding the goal location that can be recalled later.