On Finding Yourself

The hippocampus and associated structures such as the entorhinal cortex, have long been known to play an extremely important role in navigation and memory formation (as previously discussed in this forum: 1, 2). For example, the hippocampus is enlarged in London taxicab drivers, who presumably employ it heavily for navigating around the city, and individuals with damage to this area are unable to form new memories at all, though they can recall past experiences with no loss of fidelity.

The entorhinal cortex feeds into the hippocampus, and it seems to be far more specialized for navigation purposes. There are cells in this area that seem to encode the direction that an animal's head is pointing. There is another varietal, referred to as the grid-cell, whose response-properties are illustrated below.

from reference 2

Grid-cells have the intriguing property of responding vigorously in a regular array of spatial locations. If an experimenter puts an animal in a small confined space, the regularity of these responses are evident after a brief period of exploration by the animal. On the left, you see the black trace of a rat's position as it wanders around this enclosure, with red traces representing locations where the response of a single neuron under consideration was strongest. In the middle, you can see a rasterized representation of this information, and on the right, a "cross-correlation" of the middle plot, showing the regularity of the responses.

Much of the pioneering work on the hippocampus and entorhinal cortex has come from the lab of Edvard and May-Britt Moser, a married pair of neuroscientists working at the Kavli Institute for Systems Neuroscience at the Norwegian University of Science and Technology. They have recently discovered another, infrequently occurring, class of cell in the entorhinal cortex, the border cell. Predicted from theoretical considerations in the year 2000 by Neil Burgess, these cells respond preferentially when the animal is placed near a border, of a certain orientation, of an environment (these can be walls, or drop-offs; see image, below).

from reference 1

Panel A shows that the response is independent of the size of the space, and that if the space is expanded, so too does the area over which the neuron responds (right). Panel B shows that if a new separation is inserted into the space, the cell begins to respond strongly to the boundary. Panel C shows that the response properties persist even after the walls are removed, leaving only a drop-off). Panel D shows that the orientation specific quality of the responses are independent of the shape of the room. If a landmark, in the form of a marker on one of the walls, is employed, the responses rotate along with the landmark (see below). Which is impressive, but also expected and necessary for these cells to function efficiently as part of a navigational system.



It is still a mystery how these cells produce these responses. It must be the case that a computational transformation of a variety of sensory and motor information must contribute to the computation of border location. These cells represent some of the few that have such clearly defined properties. That is to say, in many other parts of the brain, single neurons contribute only a small part of the over-all response to a stimulus, and it is thus surprising to find single cells devoted to the entire border of a space. Another impressive and enigmatic feature of the responses of these cells is the fact that they must rapidly re-compute and respond to the borders of a novel place. This sort of short-time-scale neuronal plasticity is of great interest to neuroscientists everywhere.

Understanding the properties of these cells, and their role in navigation and memory formation will be a great and rewarding challenge, one that I'm sure the Mosers are up to.

References:
1. Solstad T, Boccara CN, Kropff E, Moser MB, Moser EI. Representation of geometric borders in the entorhinal cortex. Science 322: 1865-1868, 2008.
2. Hafting T, Fyhn M, Molden S, Moser MB, Moser EI. Microstructure of a spatial map in the entorhinal cortex. Nature 436: 801-806, 2005.