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You live in a mostly 2D world, but the map in your brain charts the places you've been in 3D

Place cells in the brain light up in familiar places, both on the ground and climbing in the air

Dori Grijseels

Neuroscience

University of Sussex

Most of us will get lost from time to time. But, nevertheless, our brains are actually pretty amazing at navigating. Even when you zone out on your daily commute, your brain is still able to get you there safely. This is thanks in part to your place cells.

Place cells are brain cells that respond to a particular location in the environment and become active when you are in that location. Researchers discovered them by putting rats in a square chamber and watching them run around. They monitored when and where the rats' place cells were active. By doing this, they could determine what location each place cell was responding to, called the cell's "place field." These place fields are usually circles of 15-25 inches in diameter for rats and mice. 

These experiments taught us a lot about how place cells work, but they are not a great representation of the real world. A barren, square, white box is an unlikely environment for a rat, let alone a human, to encounter in the wild. How do your place cells represent locations in a real life, 3D world?

A three dimensional (3D) lattice used to see how place cells in mice brains responded to a 3D environment.

The 3D lattice used in the experiment

Grieves et al 2019. "The place-cell representation of volumetric space in rats." 

Roddy M. Grieves, a neuroscientist at University College London, has designed a new rat navigation experiment to answer this question. He built a cubic lattice, a 3D grid that the rats could climb in any direction they pleased. They couldn’t just move along the floor, but also vertically up and down. 

Grieves and his colleagues wondered what the place fields of place cells would look like in such an environment. They posed two hypotheses. The first was that the place field might now look like cylinders, so one place cell would respond to a location parallel to the ground, regardless of high in the lattice the rate had climbed. The second was that the place fields would become spheres, so the place cell would take height into account and only respond when the rat was at a particular location along the ground and at a particular height. This hypothesis comes from research in Egyptian fruit bats, whose place cells have such spherical place fields.

The researchers discovered that place cells in rats roughly followed the second hypothesis: their place fields took the shape of elongated spheres, like rugby balls. The elongation was always along one of the three directions the rat could run in the wire frame lattice. Generally speaking, the place fields were more stretched in the vertical direction than they were horizontally. This is important, because it means that the cells are less accurate in this direction. In other words, the place cells were less precise in knowing how far up the rat was in space than where it was within the lattice on the ground. 

This may be because rats are more inclined to move horizontally than vertically. Of course they can climb, but they tend to spend most of their time walking along the floor. Their place cells may just not be optimized for vertical movement. Another reason the place cells may be worse in the vertical direction is that it is physically harder for the rats to run in this direction. This makes it more difficult for them to gauge how far they’ve traveled.

This research may give us an insight how the human brain performs navigation, because the human hippocampus, which is critical to navigation, is similar to that of rats. And like rats, we are mammals who generally navigate predominantly in a 2D environment. However, our environment is becoming more and more 3D, with tall buildings, bridges, and underground structures. 

These findings in rats suggest that our brains are mainly tuned to the direction we’re used to navigating: parallel to the ground. Since most of our world is laid out flat in front of us, like the floors in buildings, this makes sense. Even if we fly a plane or drive a submarine we are often still moving parallel to the ground. But this might not always be the case.

Different examples of three dimensional (3D) place fields from rats.

Examples of different shaped 3D place fields, with (L-R) one, two, three, or four place fields visible.

Grieves et al 2019. "The place-cell representation of volumetric space in rats."  

One possible future scenario where humans would be truly navigating in 3D would be in outer space. Without gravity there would not be a single direction that would be relatively easier or harder to move along, such as the vertical axes for the rats. As of now, we don’t know what our place cells might do in such a situation and, more importantly, whether we would be able to navigate around as efficiently as we do in our 2D world.

Both on Earth and in space the design of the environment ultimately has a big impact on our ability to navigate. We are better able to navigate if there are plenty of landmarks around to tell us where we are. We can orient ourselves more easily if a space is not symmetrical. We also navigate better through an environment if we have experienced it from different angles. 

Knowing how we find our way around, especially in complex environments, has major implications for the fields of architecture and urban design. It should also be taken into account by engineers and anyone who designs the spaces around us. The goal of research into navigation is not just to understand how our brain works, but also to use this information to make the world around us more suitable for navigation. 

This collaboration between neuroscience and urban design is combined in the burgeoning idea of ‘conscious cities’: environments built to take the needs and behaviors of humans into account. As our cities are getting larger and more complex, neuroscience research will become increasingly important in guiding their design.

Comment Peer Commentary

We ask other scientists from our Consortium to respond to articles with commentary from their expert perspective.

Alison Koontz

Neurobiology

Caltech

A really cool piece! I’d definitely be interested to see how our internal GPS changes when we  explore new environments, like space. Is the capacity for tracking vertical motion already there, just unused (is it a trait in all mammals for instance) or is it a unique development for animals like bats that already fly and need 3D navigation?

Also, slightly more off topic, I was reading an article the other day about how GPS navigation is making our brains worse at navigation. Our brains are getting lazy because we aren’t taking an active role in navigation!

Thanks for the piece, Dori!

Dori Grijseels responds:

Thanks for the comment! I think you’re asking the key question there, whether there is a capacity for tracking in any direction. Although this research in rats has come closer to answering that question than ever before, I think more innovation is needed in the environments we can test the rats in, in order to really give a conclusive answer. 

This is a great piece about a neat experiment that extends what we know about place cells from a 2D lab environment to a more realistic 3D environment. It’s interesting that a rat’s place cells are more precise in representing locations in the horizontal plane than in the vertical direction, while bats have place fields that are more spherical. It would be useful to do similar experiments with other animals that commonly navigate in 3D, like those that live in trees, and see if their place fields are similarly spherical - suggesting that their place cells represent the vertical direction just as precisely as the horizontal plane. I also thought the connection you drew to navigation in outer space was a fascinating insight into the potential challenges we don’t often consider with navigating new types of environments. Thanks for this article!

Dori Grijseels responds:

I agree it would be interesting to look at this in a wider range of species. I would be particularly interested in marine species, as they also have a natural 3D element to world they usually navigate. Largely the techniques to record brain activity from other species just haven’t developed enough yet to be able to do that sadly, but hopefully soon!