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COPYRIGHT 2006 Natural History Magazine, Inc.
For all living creatures, knowledge of the surrounding environment and their position within it is key to behavior and critical to survival. At the simplest level spatial "knowledge" may encompass no more than the ability to orient toward or away from a stimulus. In complex organisms, though, the representation of space is a cognitive process, in which inputs from several senses--sight, hearing, the sensations of motion and posture provided by the inner ear and muscle tension--are bound together. Such binding is a function of the brain. How is it accomplished?
The brain represents information about space in many of its areas and in many different ways. For some purposes the brain represents space with egocentric coordinates, that is, from the point of view of the sensing organism. For example, the brain encodes where a light is relative to the fovea of the retina, or where an odor or touch comes from with respect to the body. For other kinds of behavior the brain encodes the organism's position with respect to the outside world, and the relations of external objects with respect to one another. Such position coordinates, which are centered on the world, are known as allocentric coordinates.
The eighteenth-century German philosopher Immanuel Kant, one of the forefathers of cognitive psychology, argued that the ability to represent space allocentrically is built into the mind. People, in Kant's view, were born with principles that ordered experience in space and time, and were prepared to interweave sensations automatically within this framework in specific ways, whether the sensations were elicited by objects, melodies, or tactile experiences.
In the early 1970s, John O'Keefe, a cognitive neuroscientist at University College London, applied this Kantian logic about space to explicit memory-memory that is recalled by deliberate, conscious effort. Explicit memory, which concerns such things as facts and events, people and objects, can be contrasted with implicit memory, such as motor or perceptual skills and conditioned responses, which are accessed and performed unconsciously. O'Keefe argued that many forms of explicit memory are associated with spatial coordinates--that is, we typically remember people and events in a spatial context.
This idea is not new. In 55 B.C., Cicero, the great Roman statesman and orator, described a Greek technique for remembering words. The idea was to picture the rooms of a house in sequence, associate words with each room, and then mentally walk through the rooms in the right order. To this day some actors and others who must memorize and recall information rely on the technique.
O'Keefe was the first to realize that rats have a multisensory representation of extrapersonal space localized in a part of the brain known to be involved in explicit memory storage, called the hippocampus. In 1971 O'Keefe probed how individual neurons, or nerve cells, were activated in the hippocampus of laboratory rats, as the animals walked around in an enclosure. Some neurons, he discovered, are activated when that animal moves to one position, whereas others fire when the animal moves elsewhere. He called these neurons "place cells." On the basis of those findings, it is thought that as an animal explores its surroundings, the brain breaks down the territory into many small, overlapping areas, similar to...
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