When we navigate through our environment, do we track mostly in two dimensions or in three? Clearly, we can move in all three dimensions, but does our internal map relate to the vertical direction in the same way as it does to the horizontal axes on the ground?
That question has not yet been definitively answered, but recent research on flying bats shows us what three-dimensional navigation looks like in the brain.
It turns out that fruit bats also mostly navigate in two dimensions, since quite a bit of their flying time involves getting from their caves straight to their favorite fruit trees. But once they get to a tree, they switch to three-dimensional flitting patterns that cover the entire volumetric space around the tree.
To gain some insight into the workings of the bats’ internal mapping system, neurobiologist Dr. Nahum Ulanovsky attempted a feat no one had tried before: seeing into a bats’ brain while it was on the wing. His team spent several years developing a tiny device with electrodes that could measure neuronal activity in the bats’ brains while allowing them to fly naturally. They then let their subjects swoop and dive around some “fruit trees” – poles dangling cups filled with fruit – up inside their oh-so-cool, fully-equipped bat lab.
Image: Dr Yossi Yovel in the lab of Dr. Nachum Ulanovsky, Weizmann Institute of Science
It seems that bat neurons, at least, relate to all three dimensions equally. Their internal maps – as well as ours – are plotted in groups of neurons called place cells. These cells respond to specific locations in the spatial environment. As the bat moves though space, the corresponding place cells send off “you are here” signals. By comparing the bats’ actual locations with the neurons’ signals, the researchers found that each place cell responds to a spherical space – i.e., it has the same resolution in all directions.
Do human place cells map a similar space, or are ours flatter? On the one hand, says Ulanovsky, our nearest relatives probably have place cells very much like those of bats, since they navigate in three-dimensional space through the tree canopy. On the other hand, since we basically walk around in two dimensions, our up-and-down axis may have lost some of its resolution over time, making our internal mapping closer to that of animals that crawl rather than those that fly. This, says Ulanovsky, is a fascinating open question that awaits further experiments.