Orientation in space
The cerebral coordinate system
Maps help us to locate ourselves in unfamiliar territory – if they are keyed to local landmarks by a system of coordinates. If a grid tiles the map, each location on the map can be assigned a unique label, such as ‘D4’ , for instance. Experiments on rats have revealed that the brain’s internal coordinate system for space involves so-called grid cells.
Each of these highly specialized nerve cells divides space into a grid pattern composed of equilateral triangles and fires only when the animal approaches an intersection of these triangles. A grid cell thus fires at multiple, regularly spaced locations. But these locations carry no label; the firing pattern of a single cell will not distinguish between different locations.
A code for position
LMU researchers at the Graduate School of Systemic Neurosciences and the Bernstein Center for Computational Neuroscience have now demonstrated how the grid cell network can represent spatial relationships in a highly efficient fashion – even more efficiently than if each cell were to encode a specific location. The trick lies in the brain’s use of multiple, shifted grids of different sizes.
“The grid cell network functions rather like the decimal system,” as Martin Stemmler, the leader of the study, explains. “Some cells encode single-digit units, such as the 8 in 18, 28, or 38, while others reflect units of 10s, 100s, and so on. Such a system needs only 30 encoding units to represent 1000 three-digit numbers, and twice that number to record one million six-digit numbers. The brain uses the same ordering principle to represent spatial relationships, which makes the brain’s representation of space highly precise.” (PRL online, 2. July 2012) göd