Hippocampal place cell encoding of sloping terrain – For the people

In all mammalian brains, from humans to bats to rats, there are specialized neurons called “place cells“. These brain cells are only active when we are in a particular place within an environment. You have some place cells for your desk and some place cells for your kitchen and very active place cells for your fridge and pantry, where all your food is! All these place cells come together to form a mental map of the environment and aid mammals in navigation. What we don’t know so much about is how do these cells know when a place is difficult, such as a steep hill or uneven terrain. So this is what I set out to ask in my PhD.

Place cell animation. Source: Ryan Jones

I had rats run on a track while I recorded their place cells. When the track was tilted, sort of like making a hill, some of the place cells changed their activity in response to the new slope. However, a smaller proportion remained stable despite the change in slope. The neurons that changed may help to distinguish between the different “hills” while the ones which remain stable aid in associating together the similar features of the two different hills. These processes can help us to keep memories separate (changing place cells) while at the same time linking together memories that share features in common (stable place cells), e.g. our experiences climbing different hills.

Our most interesting finding has to do with the rhythm that these neurons fire at. Normally place cells fire at a rhythmic pace that is correlated to a mammals movement speed. However, we saw that as the slope got steeper, this rhythmic firing actually slowed down, despite the rats running the same speeds. This change in the rhythm of the place cell firing may tell the brain “hey, this environment is really hard to move across!”

The picture is some example place cells, where the warmer (red) indicates more activity while the cooler (blue) is less activity on the track across the different tilt angles.