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Brain’s map of space falls flat when it comes to altitude

8 August 2011

The brain’s ‘map’ of space may be surprisingly two-dimensional and unable to accurately register our altitude, according to new research funded by the Wellcome Trust.

In research published online today in 'Nature Neuroscience', scientists at UCL (University College London) studied cells in or near a part of the brain called the hippocampus, which forms the brain's map of space, to see whether they were activated when rats climbed upwards.

The study looked at two types of cells known to be involved in the brain's representation of space: grid cells, which measure distance, and place cells, which indicate location. Scientists found that only place cells were sensitive to the animal moving upwards in altitude, and even then only weakly so.

Professor Kate Jeffery, lead author, from UCL Psychology and Language Sciences, said: "The implication is that our internal sense of space is actually rather flat – we are very sensitive to where we are in horizontal space but only vaguely aware of how high we are.

"This finding is surprising and it has implications for situations in which people have to move freely in all three dimensions – divers, pilots and astronauts, for example. It also raises the question – if our map of space is flat, then how do we navigate through complex environments so effectively?"

How the hippocampus makes its map of space in two dimensions is fairly well understood, but the world has a richly varied topography, and a useful map therefore needs to work in all three dimensions. However, adding a third dimension makes things much more complicated for a map, and it is not clear how – or even if – the brain can encode this.

To begin to answer this question, scientists looked at neurons known as grid cells, which become active periodically and at very regular distances as animals walk around, forming a grid-like structure of activity hot-spots. Previous work has found that grid cells are largely concerned with marking out distances.

In the study, rats walked not only on flat ground but also on pegs on a climbing wall or on a spiral staircase, so they moved both horizontally and vertically. The researchers found that the grid cells kept track of horizontal distance but did not measure out vertical distances. In other words, grid cells do not seem to 'know' how high they are.

In the second part of the study, scientists looked at another group of neurons, known as place cells. Place cells, found in the hippocampus itself, produce single activity hotspots in the environment and seem to function to encode specific places. These neurons were only weakly sensitive to height; they did show some responsiveness, however, suggesting they received information about height from another, possibly non-specific, source.

Professor Jeffery said: "It looks like the brain's knowledge of height in space is not as detailed as its information about horizontal distance, which is very specific. It's perhaps akin to knowing that you are 'very high' versus 'a little bit high' rather than knowing exact height."

Image: Khumbu region, eastern Nepal. The region contains three of the world’s highest mountains, including Everest. Credit: Carole Reeves, Wellcome Images.

Reference

Hayman R et al. Anisotropic encoding of three-dimensional space by place cells and grid cells. Nat Neurosci 2011 [e-pub ahead of print].

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