Thursday, September 13, 2012

How high are you exactly?

Your brain might not be sure.
Spiral stairs at the Vatican (I took this picture)
In a study out last year, Hayman et al., (2011) investigate whether the classic place cells and grid cells of the rat brain also encode vertical height. 
We've discussed place cells before, so read this if you want to get back to the basics. Grid cells are a sort of extension of place cells.  They are cells that fire in a regular pattern over an area while you move around in it. 
like this (source)
The red dots represent when the neuron fires and the black line represents the path that the animal (probably a rat) was traversing.  As you can see the neuron fires when the rat reaches any of the points that make up a regular grid.   
But this is just the rat crawling around on a flat surface. What happens if you have the rat move vertically?  Does a vertical grid show up? Hayman et al. tested exactly that by introducing the rats to the exciting world of rock climbing.
While the rats were climbing around on this rat-sized rock wall, the cells that had fired in a grid pattern on a flat surface actually fired in a striped pattern on the pegboard. 

Figure 2A Hayman et al., 2011

On the left is the cell firing like a normal grid cell on a flat surface, but on the right a grid cell (not the same one) is firing in a striped pattern on the vertical climbing wall.
The authors suggest that this might be just the normal grid showing up but extending along the vertical plane. In other words, each point of the grid includes the space directly above and directly below it and basically forms a grid of columns. 

This finding could mean a number of things:
1. The brain does not encode vertical space very specifically.
2. Vertical space is encoded, just not in the hippocampus and entorhinal cortex (where place cells and grid cells reside).
3. A rat's brain doesn't encode vertical space, but maybe brains in other animals (flying animals for example) do.
In a mini review of this paper, Savelli and Knierim (2011), suggest that future experiments on flying mammals known to have grid cells (such as bats) would shed light on the third point. 
Vertical grid cells in 'the flying squirrel'? (source)
I agree and I also wonder if the entorhinal cortex of humans could develop three dimensional grid cells under certain conditions.  Could people who really need to know where they are in vertical space, such as trapeze artists or gymnasts, develop a more specific sense of height?
ResearchBlogging.orgHayman R, Verriotis MA, Jovalekic A, Fenton AA, & Jeffery KJ (2011). Anisotropic encoding of three-dimensional space by place cells and grid cells. Nature neuroscience, 14 (9), 1182-8 PMID: 21822271

Savelli F, & Knierim JJ (2011). Coming up: in search of the vertical dimension in the brain. Nature neuroscience, 14 (9), 1102-3 PMID: 21878925

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