Revista Scientific American Enero 2016. Inglés

Revista Scientific American Enero 2016. Inglés

ISSN: 0036-8733

Descripción

Determining where we are in relation to our surroundings streets, trees, walls or other landmarks around us remains an essential skill without which our own survival, or even that of our species, would rapidly be endangered.

Networks of cells lodged deep within the brain work together to assemble an internal mental map of our environment that enables us to find our way from place to place, as if these cells equated to a biological Global Positioning System.

Regions of the brain involved with pathfinding are also intimately connected to the formation of new memories. When these neural tracts malfunction, they can produce the severe disorientation experienced by a patient with Alzheimer’s disease.

Our ability to pilot a car or airplane or even to walk through city streets has been completely transformed by the invention of the Global Positioning System (GPS). How did we navigate, though, before we had GPS? Recent work has shown that the mammalian brain uses an incredibly sophisticated GPS-like tracking system of its own to guide us from one location to the next. Like the GPS in our phones and cars, our brain’s system assesses where we are and where we are heading by integrating multiple signals relating to our position and the passage of time. The brain normally makes these calculations with minimal effort, so we are barely conscious of them. It is only when we get lost or when our navigation skills are compromised by injury or a neurodegenerative disease that we get a glimpse of how critical this mapping-and-navigation system is to our existence.

The ability to figure out where we are and where we need to go is key to survival. Without it, we, like all animals, would be unable to find food or reproduce. Individuals and, in fact, the entire species would perish.

The sophistication of the mammalian system becomes particularly clear when contrasted to those of other animals. The simple roundworm Caenorhabditis elegans, which has just 302 neurons, navigates almost solely in response to olfactory signals, following the path of an increasing or decreasing odor gradient.

Animals with more sophisticated nervous systems, such as desert ants or honeybees, find their way with the help of additional strategies. One of these methods is called path integration, a GPS-like mechanism in which neurons calculate position based on constant monitoring of the animal’s direction and speed of movement relative to a starting point a task carried out without reference to external cues such as physical landmarks. In vertebrates, especially in mammals, the repertoire of behaviors that enable an animal to locate itself in its environment has expanded still further.

More than any other class of animals, mammals rely on the capacity to form neural maps of the environment patterns of electrical activity in the brain in which groups of nerve cells fire in a way that reflects the layout of the surrounding environment and an animal’s position in it. The formation of such mental maps is mostly thought to occur in the cortex, the brain’s wrinkled upper layers that developed quite late in evolution.

Over the past few decades researchers have gained a deep understanding of just how the brain forms and then revises these maps as an animal moves. The recent work, conducted mostly in rodents, has revealed that the navigation systems consist of several specialized cell types that continuously calculate an animal’s location, the distance it has traveled, the direction it is moving and its speed. Collectively these different cells form a dynamic map of local space that not only operates in the present but also can be stored as a memory for later use.

A NEUROSCIENCE OF SPACE

The study of the brain’s spatial maps began with Edward C. Tolman, a psychology professor at the University of California, Berkeley, from 1918 to 1954. Before Tolman’s work, laboratory experiments in rats seemed to suggest that animals find their way around by responding to and then memorizing successive stimuli along the path they move. In learning to run a maze, for instance, they were thought to recall sequences of turns they made from the maze’s start to its end. This idea, however, did not take into account that the animals might visualize an overall picture of the entire maze to be able to plan the best route.

Tolman broke radically with prevailing views. He had observed rats take shortcuts or make detours, behaviors that would not be expected if they had learned only one long sequence of behaviors. Based on his observations, he proposed that animals form mental maps of the environment that mirror the spatial geometry of the outer world. These cognitive maps did more than help animals to find their way; they also appeared to record information about the events that the animals experienced at specific locales.