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CHAPTER 1
INTRODUCTION
As researchers in human spatial navigation, we are frequently told: "I'm such a bad navigator, you should study me." Indeed, almost half of the college students in one study rated their navigational abilities as below average (Hegarty et al. 2006). One particular complaint is trouble when navigating in new places. These types of complaints fall into a category of navigational errors that we will discuss in some detail in this book: our tendency to favor habitual, well-learned routes, affecting the ability to reason about new spatial layouts. These complaints showcase how navigation is often error prone and, for many people, a source of significant frustration. They also highlight how all forms of navigation are not the same and can differ simply based on our familiarity with a route.
Difficulties navigating can lead to serious consequences for individuals with neurological conditions affecting brain function. One particularly devastating example of this is Alzheimer's disease and dementia, where patients often report difficulty navigating and a tendency to get lost, even in familiar neighborhoods (Cushman, Stein, and Duffy 2008; Kunz et al. 2015). As the disease worsens, some patients become lost even in highly familiar neighborhoods, wandering along the paths of telephone lines into the wilderness (Johnson 2010). Even normal aging involves a decline in spatial navigation, with changes in the ability to navigate using landmarks and a tendency to favor using familiar paths. Thus, errors in everyday navigation are not only a part of life but also a hallmark of neurobiological diseases and even healthy aging (Moffat 2009).
Is it possible that we as a species are just poor navigators? Evidence from studies of navigation in other species might readily seem to support this argument. Sea turtles can navigate thousands of kilometers in the ocean to search for food using combinations of ocean currents and sensitivity to the magnetic pole, and they can still manage to find their way back to the same nesting grounds (Lohmann and Lohmann 1996). Desert ants search for food at distances of up to hundreds of meters from their nest, covering a radius that would be equivalent, in human terms, to about 38 kilometers. Yet these ants, once they find food, can plot a direct course back to their nest and find it within 1 square centimeter of error (Wehner and Srinivasan 1981; Gallistel 1990). While the mechanisms underlying these nonhuman feats of navigation differ, there is little doubt that other species are capable of incredible feats of navigation, which nonetheless would appear central to their daily survival.
Still, if we consider human history, there are many examples of navigational feats that are so remarkable they might seem to better represent those of a sea turtle or a desert ant. Perhaps some of the most striking examples, which we will discuss in detail, involve humans navigating — in some cases, thousands of kilometers across the open ocean — with few or no mechanical aids. These feats will also introduce us to important and useful concepts we will use throughout this book. Our first example involves Puluwat sailors, a seafaring people in the Polynesian Islands, which in turn will help us understand the important concepts of externally versus internally guided navigation and the idea of path integration. Lest we think that Puluwat navigation represents a feat that only a highly adept, practiced, and skilled subset of our population is capable of, we will also consider the journey of the James Caird, in which stranded sailors navigated nearly 1000 kilometers to safety in a completely unfamiliar part of the Antarctic Ocean. The journey of the James Caird, in turn, will help us understand the idea of the cognitive map. Last, we discuss how exceptional navigational skills in Inuit living above the tree line, close to the North Pole, are fundamental to their survival in some of the harshest living conditions of the world.
Navigation of the Puluwat: Path Integration in Action
Puluwat is a small island in the southern Pacific Ocean that is part of a larger chain of islands known today as the Carolines. The Puluwat are renowned within the Carolines for their wayfinding abilities, which include navigating between islands separated by distances of up to 800 kilometers. Much of their navigation occurs across the open ocean with no visible islands or landmarks. In fact, recent attempts to circumnavigate the globe using no mechanical aids (spearheaded by Nainoa Thompson) involved training with such Polynesian sailors in order to perfect their techniques (Parker 2015). So what do the Puluwat know that the rest of us do not?
For centuries, the Puluwat have relied on multiple nonmechanical internal and external cues to navigate. The internal cues, which we will discuss in detail throughout this book, include using mental estimates of direction and distances over the course of their journey; we term this approach to navigation path integration. The external cues involve using the stars as a compass and other landmarks, like reefs and islands, as reference points. These achievements are quite amazing when one considers the specifics involved. The Puluwat outrigger sailing canoes are approximately six to nine meters in length and about two meters across and must accommodate groups of five to six people, including a navigator. Yet the Puluwat can navigate these boats between islands even over great distances, successfully arriving at their destination and returning home with little problem (Gladwin 1970). How is this possible? What navigational strategies do these sailors employ?
To learn to navigate, the Puluwat spend their first decades of life in an apprenticeship that focuses on one of two different schools of navigational training: Warieng and Fanur (figure 1.1A). One aspect of training focuses on learning relevant external cues: the locations of the constellations within the sky and how these change over the course of the night from sundown to sunrise. The stars serve as a basic compass system, providing Puluwat sailors with a bearing to maintain their course. For example, if a sailor wishes to plot a course to an island such as Satawal, he would use the star Beta Aquilae, which provides an approximate heading direction for arriving at this island (figure 1.1B; in our terminology, this would be approximately northwest). However, the star course they are taught also takes into consideration the ocean currents surrounding these islands. Thus, using Beta Aquilae as a navigation compass takes into account the slight push northward that will occur owing to typical currents and is thus slightly southward of the true goal. In this way, the Puluwat use the stars as external cues much like we use a compass.
Another critical aspect of the Puluwat training involves learning to use internal cues, like one's sense of direction, to navigate. This is because stars alone are not sufficient to navigate between islands. For example, the vast majority of Puluwat trips begin in the daylight (around noon), and thus the initial direction cannot depend on using the stars to determine bearing. Instead, this is where the use of internal cues becomes important. The Puluwat use another island that they have mentally located (but that they cannot see) to determine the direction in which to head out from their home island. This method is referred to in Puluwat as etak — roughly the equivalent of a term that we will use throughout this book: a cognitive map. Put simply, a cognitive map is a mental representation of the position and spatial relations among multiple landmarks in the external world (Tolman 1948; O'Keefe and Nadel 1978; Sholl 1987).
After having determined their initial bearing based on their knowledge of the relative positions of islands from their cognitive map, navigators must then be sure to plot as straight a course as possible. As the navigator departs with his crew, he looks behind him at prominent landmarks on the island — in this case, using external cues to validate his sense of internal heading. Plotting a course to one island will involve sighting the position of himself relative to several different landmarks on the island, which must line up precisely based on the navigator's viewpoint as the boat heads out to sea (figure 1.2). Using a landmark to guide our navigation, either toward or away from that specified landmark, is termed piloting. In this case, piloting involves using the position of multiple landmarks relative to the sailor to plot his exact angle of departure, which we term egocentric navigation because he bases his estimate of direction and distance relative to himself and landmarks. In this way, the sailor can estimate fairly precisely his angle of departure based on his memory for the view angles corresponding to relative bearings.
Once the island is out of sight, the Puluwat navigator must be sure to maintain his course and not to veer too far from his initial bearing. Here is where his ability to path integrate, or keep track of both direction and distance traveled, becomes most important (figure 1.3). From his position on the boat, the Puluwat navigator tries to maintain a specific bearing based on his internal sense of how much the boat has turned. The Puluwat have detailed knowledge of different types of waves that present within the Caroline Islands, some of which originate from the north and others from the east. When one of these waves hits the boat, by determining the angle at which the main boat and outrigger hit the wave, the navigator can estimate direction and update his sense of direction. In this way, the Puluwat sailor uses a keenly developed internal representation of bearing and then updates his course based on information he obtains from knowledge of waves that hit the boat at different angles.
In addition to bearing, the navigator also computes distance traveled based on his estimate of the speed of the boat and the time of day. Again, he uses external cues to update and correct this estimate. Based on the time it takes for the crests of two different waves to pass, the sailor can estimate the relative speed of the boat. The Puluwat sailor thus uses these various external cues to update and estimate his internal estimate for direction and distance. By having an internal representation for both direction and distance, which the sailor continuously updates based on external cues, he is able to maintain a fairly good idea of how far he has traveled and in what direction.
Path integration, though, would not be particularly helpful to the sailor unless he had some idea of where islands were located as he traveled. Recall that the Puluwat, upon beginning the journey, have a precise idea of their initial bearing based on their representation of their home island relative to other islands in the Carolines — what we have termed the cognitive map. This knowledge of the relative positions of other islands and landmarks is what we term an allocentric form of navigation and reasoning because it is based on the relative position of multiple external landmarks to each other independent of the navigator. Thus, as a sailor navigates to a distant island, he uses a reef or another island that is not visible but can be pictured in his mind's eye based on his estimate of progress from his path integration representation.
Confirmatory evidence of the position of an island or a reef can be obtained by seeing it (from about 16 kilometers away) or, in the case of an island, by sighting seabirds, which can range up to 64 kilometers from a given island. In this way, the cognitive map serves the important function of combining path integration information with the expectation of different landmarks the sailor will encounter (see figure 1.3). Encountering these landmarks, which can occur in a manner either consistent or inconsistent with path integration representations, provides an indication either of success or of the need to make a slight adjustment to course. The final landmark the sailor looks for is the destination island — his goal — at which time the sailor relies on piloting by simply using the island as a visual aid and correcting course accordingly until the island is finally reached.
What We Can Learn from the Puluwat
For anyone not accustomed to sailing and traveling by sea, even getting a boat out of a harbor might appear challenging and certainly requires a fair amount of skill; the idea of navigating hundreds of kilometers in the open ocean with no obvious visual cues seems, at first, impossible. As we discussed earlier, however, the Puluwat make use of a wealth of cues, both external and internal, that most of us are probably unaware are even useful for navigation. As we will see in chapter 2, we use similar estimates of direction and distance during walking in new and familiar environments, and, just like the Puluwat, we correct our internal estimates based on evidence from visual features. In this way, we can think of navigation as an inherently multisensory integration process that combines multiple cues to accurately find our way to our goal (Berthoz and Viaud-Delmon 1999; Angelaki and Cullen 2008).
But what is perhaps most striking is the Puluwat's highly cultivated and sophisticated use of an internal tracking of direction and distance: their path integration system. It may seem difficult to believe that our brain has a built-in system for estimating the direction we are traveling and how far we have traveled. But this is precisely what path integration, at its core, involves. Right now, you are probably sitting in a chair or lying on a couch reading this book. Try standing up for a moment. Now, close your eyes and try walking to a location that you can picture in your head, like another chair in your living room. Stop when you think you have reached your destination. As you walk, you will probably have a fairly good sense of how far you need to travel and whether you need to take any turns to get there. Hopefully, you have experienced the sense that our brain can, in fact, keep track of both direction and distance, even in the absence of any visual cues to confirm that our internal system was correct.
Computing Head Direction
We may often forget about what some call our "sixth sense," our vestibular system, because its specific contributions to everyday life may not be obvious (Wolfe 2006). Our vestibular system, an intricate series of fluid-filled canals in our inner ear, is critical for functions like balance and updating our eye position with head movements. Perhaps most important for our current considerations, our vestibular system is also critical to tracking our bearing by updating our brain about changes in our head position. As an example of its importance, lesions to our vestibular system produce profound deficits in navigation (Russell et al. 2003; Brandt et al. 2005).
Our vestibular system works much like a level, a tool frequently used by carpenters to estimate the angle of a board or other object (figure 1.4A). A level is usually a long, straight bar containing one or more tubes filled with water and an air bubble, with marks indicating the center position. If a carpenter wishes to determine whether an object is level or at an angle, she can position a level relative to the object. If the bubble deviates to the side, she knows that the object is not level. If the bubble is even relative to the center marks, she knows that the object is level.
The vestibular system works in much the same way, except it contains vessels for each of three different perpendicular directions. These curved canals contain a watery fluid that moves, or is displaced, every time we move our head. The three canals that detect these rotations are termed semicircular canals (figure 1.4B). Because each semicircular canal is perpendicular to the other two, the three curved canals cover all three possible primary orientations in three-dimensional space. Thus, movement of your head in any direction in three-dimensional space will activate at least one of your semicircular canals, allowing you to detect displacement in almost any angular direction.
Within each semicircular canal, tiny hairs called cilia detect the movement of this fluid. The hairs act much like seaweed does when a wave passes over it in the ocean. Just as the seaweed bends to follow the direction of the current's movement, the hair cells in our semicircular canal are pushed by the movement of the water in our inner ear. For example, when you shake your head from left to right and back, fluid in your semicircular canals moves hair cells in your inner ears in opposite directions. Based on the rate at which the fluid deflects the hair cells, we now have a way of computing the angular acceleration of our head movement. When we turn our head, either with our body or independent of our body, our semicircular canals, via our hair cells, provide information about how quickly we moved our head and when we stopped. Neurons in our vestibular nerve code this displacement by a change in the activity of neural signals called action potentials, which we will discuss in greater depth later. These, in turn, provide a signal for the acceleration of your head in one direction versus another.
(Continues…)
Excerpted from "Human Spatial Navigation"
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