There was a recent post in the Neurophilosophy site about our brains way of handling space (here), Rats know their limits with border cells.
Spatial navigation is the process on which we rely to orient ourselves within the environment and to negotiate our way through it. Our ability to do so depends upon cognitive maps, mental representations of the surrounding spaces, which are constructed by the brain and are used by it to calculate one’s present location, based on landmarks in the environment and on our movements within it, and to plan future movements.
We now know that the circuitry encoding the cognitive map lies in the hippocampus and surrounding areas, and that these parts of the brain contain at least 3 distinct types of neurons which together encode an organism’s location within its environment and the paths it takes to move through it. In the current issue of Science, researchers from the Norwegian University of Science and Technology in Trondheim report that they have discovered a fourth class of neuron involved in spatial navigation.
Research into the cellular basis of spatial navigation began in the early 1970s, with the discovery of place cells by John O’Keefe and Jonathan Dostrovsky. Place cells were initally found in the hippocampus of the rat, and have since been found in other organisms, including humans; each fires only when an animal is in a specific location within its environment. Then, in 1984, James Ranck of SUNY Health Sciences Center in New York identified head direction cells in the presubiculum, which is adjacent to the hippocampus. As their name suggests, these neurons fire only when an animal is facing a certain direction.
The third type of neuron involved in spatial navigation is the grid cell, which was first identified in 2005, and is found in the entorhinal cortex, which also lies next to the hippocampus, and in rodents is located at the caudal (back) end of the temporal lobe. Unlike place cells, grid cells fire when the animal is at multiple locations in its environment. These locations are evenly spaced, so that a grid cell increases its firing rate periodically as the animal traverses a space. . Grid cells encode different scales, such that small groups of grid cells have a unique periodicity; this scaling is mapped onto the entorhinal cortex, so that the scale encoded increases systematically along its top-to-bottom axis.
In a paper published in 2000, Neil Burgess predicted the existence of what he called boundary vector cells, which encode the organism’s distance from geometric borders surrounding its environment. The prediction was based on a computational model of place cell activity, but until now there has been no experimental evidence for such cells. Edvard Moser and his colleagues, who first described grid cells in 2005, now confirm the existence of these neurons in the rat brain . they found that the firing rates of the cells increased only when the animals were at one or several of the walls of the enclosure, irrespective of the length of the border or its relationship with other borders in the surroundings the cells are responsive to borders in general, and not just walls. Moser suspects that border cells align grid cells to borders, and are thus involved in defining the perimeter of the animals’ environment. He also suggests that they are involved in route-planning – although they are sparse in number, they are distributed widely throughout the entorhinal cortex, in such a way that they could provide grid cells with information about approaching obstacles and borders.