Wayfinding
Page 8
“Critical periods are when the system is particularly sensitive—if it doesn’t receive the right stimuli, it is stunted,” said Alessio Travaglia, a postdoctoral fellow and author of the study. “The brain is maturing through experience. We do think that without the right stimulation the hippocampus is not going to develop, actually. It’s not just the infantile amnesia, the fact that there is a critical period in maturation we think has big implications for education and what kids need.” He used the example of an eye. “This was an early experiment done in the sixties. If you put a patch on your eye and keep it closed for a week, after a week you can still be fine. If you close the eye of a young animal early in life during this critical period, they found the animal cannot see, it’s going to be blind. Another example of a critical period is language. For example, if a baby learns another language when it’s very young, it’s going to be fluent.”
Travaglia and his fellow researchers think the hippocampus needs experience and opportunities to mature. “For humans, the assumption is that the brain needs the right stimulation in this critical period too. The right stimulation, meaning that kids should experience the right noise, games, environment, playing, and if they lack these stimuli they might have an effect later on,” he told me.
One possible developmental milestone is when a child transitions from being passively carried to self-mobile. Perhaps it’s this change in movement that impacts how spatial information is encoded in memory? In 2007, for example, a group of researchers in England found that the onset of crawling in nine-month-old babies was associated with a cognitive leap: a more flexible and sophisticated capacity for memory retrieval. Arthur Glenberg, a professor of psychology at Arizona State University, has a hypothesis that the onset of self-locomotion prompts hippocampal maturation: once babies begin moving through space on their own, their place cells and grid cells can begin aligning themselves to the environment, ultimately facilitating the creation of the infrastructure of long-term memory. He thinks the tuning of these cells depends on the consistent correlation between optic flow, head direction, and the unconscious perception of spatial orientation from self-generated movements; until infants begin moving themselves through space, the whole system is immature and therefore an unreliable contributor to memory. When infants begin to crawl and explore space, conditioning the spatial location coding, that movement can become the scaffolding for long-term episodic memory, and forgetting declines. Glenberg’s hypothesis also provides an intriguing explanation for memory degradation in older age: as the body ages there may be less self-locomotion and exploration. Perhaps, he suggests, hippocampal place and grid-cell firing become dissociated from the environment and result in less powerful memory recall.
Glenberg’s idea doesn’t fully explain why there is such a large gap between the start of self-locomotion in the first year of human life and the reliable retention of memories that takes place around age six. He suggested that the tuning of the hippocampus to the environment by crawling has to be relearned once we begin walking. But it’s also possible that this gap comes down to experience and how much is required. It takes time to explore enough space and begin formulating complex cognitive maps and a fully functioning hippocampal memory system anywhere near the sophistication of an adult’s. In fact, the age of self-locomotion seems to matter less than the amount of exploration children engage in. Dutch researchers found in 2014 that by the age of four, children who spent more time exploring had a higher capacity for spatial memory and a positive correlation to fluid intelligence—solving problems, identifying patterns, and logic. “Your ten-month-old knows his way around the apartment but would not have much luck getting from the apartment to the park,” Glenberg told me. “It takes an awful lot of experience walking around to develop this complex-enough set of cells that can serve as a good substrate for memory.”
In 1999, a group of researchers led by Rusty Gage at the Salk Institute for Biological Studies in California discovered that exercise induces neurogenesis in the adult hippocampus, specifically the dentate gyrus, the region through which the hippocampus receives most of its connections from other parts of the brain and which is implicated in forming episodic memories. More recently, three researchers at the National Institutes of Health’s program on aging looked at the brain cells of adult mice that spent a month with running wheels in their cages, a group that had wheels for one week, and a group that had no wheels. Afterward, both groups with wheels had brains that had developed new neurons and longer dendrites compared to mice that had no wheels. Running, the researchers concluded, likely facilitates spatial information encoding by increasing the generation of neurons while also reorganizing neuron circuitry.
The fact that hippocampal development is influenced by these kinds of activities and experiences indicates an incredible plasticity, with implications for childcare, education, and the treatment of cognitive impairments. “It’s very exciting because the maturation of the brain is often considered dependent on time and a genetic program,” Travaglia told me. “What we’re showing is that the development of the brain is not a fixed program, it’s about experience.”
* * *
In the 1940s the psychologists Jean Piaget and Bärbel Inhelder tested children on a “three mountain task.” They placed a doll on different parts of a small-scale model of three mountains and asked the children to select one of several pictures to match the doll’s perspective at each spot. At four years of age, most of the children couldn’t distinguish their point of view from the doll’s, leading the psychologists to believe that young children rely on the more elementary egocentric perspective that precedes logical thinking. Later, around nine or ten years of age, children, they thought, switch to an allocentric representation, the ability to encode the Euclidean, objective relationships between landmarks and assume the perspective of multiple objects to each other.
Later research has shown this classic development sequence from egocentric to allocentric in children to be flawed: Newcombe has shown that babies as young as twenty-one months can accurately represent locations allocentrically. In a 2010 study published in the Journal of Experimental Child Psychology, Norwegian and French psychologists tested seventy-seven children in elementary school using a virtual maze task. They found that while all of the five-, seven-, and ten-year-olds used a sequential egocentric strategy to solve the task, they were able to adopt an allocentric strategy too. Even the youngest chidren tested could do it. But, the older the children were, the more spontaneously they could transition to the allocentric perspective and use it with greater accuracy; ten-year-olds were able to orient themselves at the beginning of the task and create an abstract top-view representation of the maze equal to that created by adults.
The findings suggest that while young children are able to employ the allocentric strategy, the nature of it changes progressively between five and ten years of age. By ten, individual children can demonstrate startling variations with their peers in hippocampal volume. Researchers have found that children who have higher levels of physical fitness have larger hippocampal sizes than those who are less active, indicating a relationship between aerobic exercise and the structure of preadolescent brains. Furthermore, those structural differences seem to impact function. The same ten-year-olds who were more physically active and fit showed better performances on memory tasks.
We’re not the only animals that demonstrate the plastic nature of the hippocampus and its relationship to cognitive ability. In nonhuman primates, hippocampal volume is the measure most consistently correlated with performance on spatial and nonspatial tasks, and can even predict performance. University of Oxford researchers Susanne Shultz and Robin Dunbar have looked at forty-six different species of primates, including gorillas, lemurs, and macaques, and given individuals eight different tasks designed to test learning, memory, and spatial cognition. Those primate species with a larger hippocampus performed better. Proportional brain size in primates has been found to correlate with social learning
and tool use, the formation of coalitions, the ability to deceive, and the size of social groups, all aspects of higher-order cognition or what is also called executive functions—the ability to organize thoughts and actions and direct oneself toward obtaining goals. The demand for increasingly sophisticated executive functions among primates may be one of the selective pressures that actually led to their brain enlargement (and, eventually, us).
Shultz and Dunbar have also discovered that birds that store food in different locations, sometimes returning days or months later to retrieve it, have a bigger hippocampal homologue. In one of their earlier studies in the late 1980s, they chose thirty-five different species and subspecies of passerine birds, an order that encompasses over half of all bird species who use their toes to perch, and dissected the brains of fifty-two specimens taken from the wild. Some of the birds belonged to species known to store food, and some relied on foraging alone. The researchers wanted to know: did food-storage techniques place greater demands on memory? And did birds that used these strategies develop special memory capacities that might be reflected in their brain volume? Shultz and Dunbar found that a bird like the marsh tit, which stores food in woods, had 31 percent higher volume in its hippocampus than a closely related bird, like the great tit, that forages.
Seven years later, Shultz and Dunbar decided to look at a single species, the common khaki-colored garden warbler. Would garden warblers with more migration experience have a larger hippocampus than those that didn’t? If so, perhaps they were not unlike those taxi drivers whose memorization of the streets of London leads to a larger volume of hippocampal gray matter. They compared the brains of young birds that hadn’t undertaken the annual migration from Europe to Africa to those that had, and found that birds with more migratory experience had significantly larger hippocampi—the result of both greater age and more experience. Other studies with pigeons have shown how their hippocampus is important for learning landmarks, and if researchers give them lesions in this part of the brain, they lose their ability to home.
Black-capped chickadees will not only return to places to retrieve hidden stocks, but they’ll visit the stocks that hold their favorite food first and their least favorite last. This memory feat is surpassed by the humble scrub jay, which can remember not only where events happened but also when. The birds’ favorite food is wax worms, but only when fresh; once they dry up, the worms are less delectable. Two researchers, Nicola Clayton and Anthony Dickinson, conducted a study in which they gave scrub jays wax worms to hide and, four hours later, they gave the jays the choice between retrieving the hidden worms or peanuts. But in some cases the researchers waited five days after they hid the worms to give the birds a choice. After four hours the jays chose to retrieve worms, but after five days they chose peanuts. They not only remembered what they had hidden but when they had done it. So do scrub jays have episodic memory?
The difference between humans and other animals and our cognitive abilities seems to have less to do with size and more to do with the sheer number of neurons that we develop and, crucially, where in the brain these neurons are located. An African elephant’s brain is three times larger than ours and has three times the number of neurons. But its hippocampus has less than 36 million neurons compared to the 250 million neurons in ours. Nonetheless, some African elephants have been known to inhabit home ranges of over twelve thousand square miles; what specialized coordination of spatial memory and senses allows them to navigate these spaces? Some have surmised that they must use a hippocampal-dependent spatial strategy similar to humans. Meanwhile, whales also travel thousands of miles but have unusually small hippocampi and no detectable adult neurogenesis.
Pondering how animals experience the world stretches our imagination. The scientist Jakob von Uexküll thought that the behavior of an animal could only be explained by considering the inner, sensory world it inhabits. According to him, organisms inhabit their own Umwelt, a German word that means “environment,” and he used this concept to explain how animals’ subjective sensory experiences evolved to meet their needs. According to this idea, bees live in an ultraviolet world, because it allows them to orient themselves by polarized light, and wolves inhabit a landscape of smell in order to create landmarks and maps of places of import. Maybe the indigo bunting is blind to stars of lesser magnitudes in order to worship the North Star, its compass.
When it came to conceptualizing the intertwining relationships between organisms and their environment, Uexküll turned to the metaphor of music. Every organism is like a melody that resonates and harmonizes with living things around it. As he wrote, “All living beings have their origin in a duet.” For children, that duet seems to be the interaction between the neurons firing in their mind and the places where they grow up.
BIRDS, BEES, WOLVES, AND WHALES
One morning in the Arctic I woke early and pulled on thick, windproof pants and an anorak with a hood edged in wolf fur and climbed over still-sleeping adults and children spread across the floor of a one-room cabin. I opened the plywood door a crack and snuck through it so as not to let the freezing air inside. Shoving my feet into heavy, felt-lined boots, I stood up and took in the sight before me. The cabin sat high on a hillside at the mouth of a large inlet covered in aquamarine sea ice that had been pushed by powerful tides into the coastline to form giant ruches. To get to this remote cabin we had sledded south from Iqaluit for hours, hugging an edge of the bay, past a place called Pitsiulaaqsit, which means the island where guillemots nest in Inuktitut, and another called Qaaqtalik, meaning the place where a mattress of caribou skins was left long ago. We had turned inland at Nuluarjuk, the island shaped like small buttocks. After spiking a long chain onto the ice and tying the dogs to it, we collected blocks of frozen freshwater from a nearby pond up in the hills, cutting them away with heavy metal spades and melting them to drink. For dinner we ate aged caribou ribs, delicate pieces of raw Arctic char, ptarmigan seared in its own blood, and boiled musk ox. Below I could see the teams of dogs where they had slept the night; they barely lifted their tucked noses to acknowledge my presence, and beyond them I saw the bay, a frigid expanse of white. The floe edge, where the ice meets open ocean, was still several hundred miles south of us.
I’d arrived on Baffin Island expecting most hunters to get around with dogsleds, but I quickly realized this was akin to showing up in New York City and wondering where all the horse-pulled carts were. While Greenlandic hunters are required by law to use sled dogs for hunting, and some remote communities in Nunavut still maintain sled dog teams for racing, in the whole of Iqaluit there were only about half a dozen teams. Everyone else used snowmobiles. I’d managed to ride to the cabin with a team of dogs belonging to Matty McNair, an explorer who had captained the first all-female expedition to the North Pole and has lived in Iqaluit for decades. Her dogs were well conditioned and had traveled all over Baffin Island, often on trips where McNair intentionally navigates using mainly landmarks, stars, and snow. But her dogs, she told me, are far better at finding their way than she is. “I don’t know how they navigate: I’ve had dogs hit the town dead on in weather I couldn’t even see in,” she said. “It wasn’t smell, you couldn’t see anything visually, there were no trails they were on. It’s just absolutely uncanny how they navigate. I’ve also been out at the beginning of the year and going on a snowmobile trail, and the dogs will turn off and go around the rock because that’s where the trail went last year. They don’t care where the snowmobile trail goes, that’s where the trail went last year.”
The Inuit sled dog is a distinct breed that made life possible in the Arctic for generations. Without them, travel on snow and ice used to be impossible; in the summer and fall they carried food and supplies across the uneven tundra. Dogs were so important that they were often fed first, then children, then adults. Today snowmobiles have undeniable advantages over dogs, which require nearly year-round hunting in order to feed them. As one dog team owner explained to me, to maintain a team of ni
ne dogs, he has to provide four and a half tons of walrus and seal meat each year. For most hunters, the time required to feed dogs is simply too much of a burden. As the dogsledder Ken MacRury told me, “Inuit are pragmatists, they are not romantics. If the dog team is not useful, they’re gone. Snowmobiles made it possible for people to keep full-time jobs and still be hunters. You didn’t have to feed a snowmobile all summer.”
The differences between riding a snowmobile and a dogsled are obvious. The former is much, much faster. But the slow speed of sledding provides an ideal pace for teaching and learning geographical and environmental knowledge, committing to memory landmarks, details of routes, place-names, and vistas. “The faster you traverse the land, the less observant of it you become,” explained John MacDonald, a resident of Igloolik for twenty-five years who worked closely with the community’s oral history project and is the author of The Arctic Sky. He once traveled with an elder in Igloolik who stopped at a rock and recognized it by the pattern of lichen on its surface. “I could have passed it and not even looked twice,” said MacDonald, “which is exactly what you tend to do with snowmobiles.” Snowmobiles also create an experience of always driving into wind, whereas hunters on dog teams travel slowly enough that they can use wind direction as an orientation tool to keep a bearing. Indeed, hunters across the eastern Arctic often used a wind compass with Uangnaq and Nigiq as the axis, and had up to sixteen terms to describe the in-between bearings. Often the Inuit dogs themselves played an important role in how people navigated, as McNair had described to me. A good driver (never called a “musher” in the eastern Arctic) rarely used a whip, if at all. The optimal relationship between the driver and his team is based on the concept of isuma, an Inuktitut word that means something like “mind” or “thinking,” and in certain contexts it can mean “life force.” The driver guides and directs his team using his mind, focusing his will on the team. The lead dog is the isumataq, which means “the one who thinks,” and is the most responsive to the will of the driver. “You have to direct your isuma,” explained MacRury. “You have to project your thoughts onto the dogs and they have to respond to that.… [Y]ou communicate with your dogs with your mind and your voice.”