"Researchers have conducted a number of experiments to determine whether humans have a magnetic compass sense, but these have been inconclusive. Expert opinion is fairly unanimous that there are no convincing indications of such a sense in humans. On the other hand, there is good evidence that many insects, birds and reptiles have such a sense, although it remains unclear under which conditions they make use of it. Also, there have been many deeply puzzling difficulties in replicating the results that document the existence of the magnetic compass sense.
"Having a magnetic compass sense is not equivalent to having a homing instinct, because knowing which way is north does you no good if you do not know whether you are north, south, east or west of home. Many animals, including humans, keep track of where they are (and hence the direction to home) by a method known as dead reckoning: as they move about, they keep track of each individual movement, adding these up to derive their net change in position.
"Dead reckoning is no help, however, when people or animals are displaced under conditions in which it is impossible for them to determine the speed and direction in which they are moving. Nevertheless, many animals--most notably homing pigeons--are able to figure out where they are even after this kind of displacement. How they do so remains a mystery, despite much experimental work on this problem. A recent special issue of the Journal of Experimental Biology (January 1996, Vol. 199, No. 1) is devoted to animal navigation. It contains reviews by many of the leading researchers."
Patricia Sharp, an expert in neuroanatomy at Yale University, adds a perspective based on her studies of navigational systems in rats:
"It is not yet clear exactly what kinds of unique navigational systems humans may have. The question is difficult to sort out, because humans can bring so many advanced, learned cognitive skills into play when solving a navigational task. My own work involves the study of brain systems in rats that seem to be involved in navigation. I suspect that humans have similar systems--but at present there is no evidence to support that suspicion.
"Work from my laboratory and others has revealed two basic kinds of spatial signals thought to be critical for navigation in rats.
"1) Head-direction cells: When a rat is placed in a large recording chamber and allowed to forage for food, there are cells in some parts of the brain that fire whenever the rat faces one particular direction. It does not matter where the rat is in the chamber; whenever it faces in that direction, those cells fire like crazy. Each of the head-direction cells has its own, unique directional preference, so that the direction the rat faces at any given moment is signaled by the firing of some particular combination of these cells.
"One might guess that these orientation cells use some sort of information about the earth's electromagnetic field in order to keep track of direction. The evidence suggests this is not the case, however. It turns out that if a rat is placed in a familiar environment and the experimenter rotates the salient environmental landmarks, the cells' preferred directions also rotate. This finding suggests that, just like us, rats use landmarks to 'get oriented.'
"When a rat is moved from a familiar environment into an entirely new one, each head-direction cell retains its directional preference. This, too, makes sense intuitively. If you walk from a familiar part of town into an unexplored neighborhood, you can keep track of your directional heading for some time just by logging your own movements: if you were walking north when you started out and you have not made any turns, for example, you must still be walking north. It is possible that the head-direction cell system is responsible for this ability, given that data have shown that these cells use information about the animal's own turns.
"2) Place cells: These neural cells are the complement of head-direction cells. They fire whenever the rat is in a certain location, rather than a specific direction. Studies indicate that place cells, too, use both landmarks and path integration (a record of the animal's movements through space) to keep track of where they are.
"It is not yet clear how these two types of cells play a role in navigation--but is seems very likely that they do.
