When you first move to London it’s very common to quickly gain very detailed, even intimate knowledge of two or three locales, but not know how they are connected geographically.

It’s not until there’s a Tube strike and you have to cycle or take the bus, or for some other reason find yourself driving or walking with central London, that you suddenly realise that places you thought were separated by several sets of escalators and two Tube lines are only 15 minutes walk apart. It was only last week that one of us realised that Goodge Street is a short walk from Euston Station… and for years the other thought that Stratford, apparently due east on the Jubilee Line, was somewhere near Colchester…

A study published last week by Francis Carpenter and his colleagues at UCL shows how this kind of spatial understanding is represented in our brains.

There are several cell types essential for mammals to navigate. For example, there are direction cells, which fire when we are facing a particular direction; place cells, firing only in particular locations in our environment; and grid cells, which fire at regular space intervals as we move about.

Grid cells are the navigational stars of the Carpenter study. Discovered by May-Britt Moser and Edvard I Moser in 2005, grid cells provide us with an internal coordinate system that enables us to find our way. Grid cells live in the entorhinal cortex, a brain region associated with navigation and memory, and their firing rates are thought to help inform us how far we have travelled and the path we have taken.

Carpenter and his team monitored grid cells in rats’ brains. They found something very interesting.

First, they put rats in identical compartments joined by a corridor (see diagram).



Apparatus: two identical compartments joined by a corridor.

Then, while the rats were navigating around each compartment, the researchers monitored their grid cell firing patterns, using electrodes implanted in the rats’ entorhinal cortex to record the activity of individual neurons.

The researchers compared the rats’ grid cell firing rates between the two compartments. They visualised the rats’ grid cell activity in each compartment by creating grid cell activity maps which reflect the locations at which a grid cell fired as a rat moved about.

Initially, the rats’ entorhinal grid cells formed two similar firing rate maps: each map represented the rats’ neural responses to environmental cues in one compartment. The maps were almost identical because of the likeness of the two spaces. However, after some time, the similarity between the two maps decreased.

The researchers noticed that the grid cell firing maps in both compartments had changed to reflect a single continuous representation that spanned the two environments (see diagram).

Initially: grid cell firing patterns replicate, reflecting identical sensory cues. After experience: firing patterns for a single, continuous grid, reflecting absolute positions in a global map.

They theorised that when the rats understood the geographical relationship between the two compartments, their individual local maps became part of a larger, global map. When comparing the grid firing patterns recorded to idealised data for local and global mapping, these observations were confirmed – the local maps initially built by grid cells were slowly transformed into global maps of the rats’ larger scale environment.



In other words, the rats came to realise that there was a rodent maze equivalent of the secret staircase at King’s Cross...



Carpenter and his colleagues speculate that the new map that was formed is permanent, allowing mammals to have this new information at hand when planning for future journeys.

But how do grid cells actually work?

Grid cells represent our location in space through a triangular coordinate system. Imagine the floor in front of you is tiled with a mosaic of interlocking triangles. As you reach the corner of any of those triangles, a particular grid cell fires.

MC Escher has a bad day. Photograph: Pixabay

The coordinate map that grid cells make up is composed of multiple layers, each with a different periodic distance between grid points. So the distance between points on each grid is fixed, but differs across layers. The combined information from different grid cells layers can then identify our unique location.

The firing pattern of a rat’s entorhinal grid cell as it moves around a room. Overlaid in blue is the triangular-like grid pattern for this cell. Image: Moser MB et al., Cold Spring Harb Perspect Med. 2015. 5(1):a021808.

What could this mean for us humans?



For one, becoming aware of the short walk between two tube stations could merge two distinct neural maps of these different locations into a single global map of the wider area – perhaps enabling commuters to plan journeys more wisely in the future.

Grid cells cleverly link places up in our minds as we move between locations, and in this way may be vital to our navigation on a larger scale. As we are beginning to understand the neural representation of space, our understanding of the fundamental mechanics of the brain and how it computes complex information is growing.

Carpenter and his colleagues are now enthusiastic to study how our global maps are used to plan journeys and take shortcuts. Knowing that Westminster is really on the west and south of Waterloo is only part of it.

Have you had a geographical ‘Aha!’ moments recently? Tweet @rpg7twit or @brainfreezemee and help us expand our global maps. #AHA!



Carpenter F et al. Grid Cells Form a Global Representation of Connected Environments. Curr Biol 2015 In press. http://dx.doi.org/10.1016/j.cub.2015.02.037