Benjamin Edwards, The Pusan Experience, 2002 



that ordered experience in space and time, and 

 were prepared to interweave sensations automati- 

 cally 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 neu- 

 roscientist at University College London, applied 

 this Kantian logic about space to explicit memory — 

 memory that is recalled by deliberate, conscious ef- 

 fort. Explicit memory, which concerns such things 

 as facts and events, people and objects, can be con- 

 trasted with implicit memory, such as motor or per- 

 ceptual skills and conditioned responses, which are 

 accessed and performed unconsciously. O'Keefe ar- 

 gued that many forms ol explicit memory are associ- 

 ated 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 re- 

 call 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 hippocam- 

 pus. In 1971 O'Keefe probed how individual neu- 

 rons, or nerve cells, were activated in the hippocam- 



pus 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 else- 

 where. 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 a mosaic, thereby forming an inter- 

 nal map. The map develops within minutes of the 

 rat's entrance into a new environment. Under opti- 

 mal circumstances, it lasts weeks or even months. 



I began to think about the spatial map in 1992, 

 wondering how it is formed, how it is main- 

 tained, and how attention might direct its forma- 

 tion and maintenance. I was struck by the finding 

 of O'Keefe and others that the spatial map of even 

 a simple locale does not form instantaneously. In- 

 stead, it forms over a period of between ten and 

 fifteen minutes after the rat enters the new envi- 

 ronment. The delay suggested that forming a spa- 

 tial map is a learning process, in which practice 

 makes perfect. Thus even though the general capa- 

 bility for forming spatial maps that Kant envi- 

 sioned may be built into the brain, each particular 

 map of a specific environment is not. 



When my colleagues and I began studying spatial 

 maps, nothing was known about the molecular de- 

 tails of their formation. But we did have a research 

 advantage. We had spent many years teasing out 



March 2006 NATURAL HISTORY 133 



