After exploring Paris for two weeks, I have come to love the sights, smells and tastes of its beautiful streets. I already found my favorite routes for traveling to class and finding new restaurants. I know where to go to find a quiet garden to study in, or a quaint little restaurant to have the most amazing gnocchi dinner. As I continue to wander around, my understanding of the layout of the city is coming together to form a cohesive picture. As any naïve tourist would report, I felt like a natural Parisian after my first successful solo excursion on the metro. And as that same naïve tourist would inevitably soon discover, I was not nearly as good at navigating as I assumed.
On one of my first days in Paris, I was lucky to be able to spend the afternoon with my boyfriend who was touring around Europe. At the end of the day, I wanted to go to a small restaurant in Montmartre that I visited over three years previously. I assumed I would easily be able to figure out the way once we got to Montmartre.
When we arrived at the metro line to go up to Montmartre, we found that it was closed, and a sign directed us to the next stop along the same line. We walked over and to our frustration, that stop was closed as well, with a sign directing us back down to where we had come from. Not ready to be discouraged, we decided to get on a different line at that station, and try to make connections until we were near our destination. Over an hour later, we found ourselves standing on an unfamiliar street, without wifi or any real idea of where we were. We oriented ourselves to walk north, since we had no better plan for how to begin.
Upon reflection, I realized that the only reason I managed to eventually eat dinner that night, was because of my ever-resourceful hippocampus. Well known for its role in memory consolidation, the hippocampal formation is now recognized to be highly involved in spatial navigation, in part due to the presence of grid cells and place cells (Jacobs, et al., 2013). Oscillations in place cell firing occur in a cycle called the theta cycle, to help orient oneself in space (Wikenheiser & Redish, 2015). Unlike place cells, which only fire for very specific locations in space, grid cells in the entorhinal cortex represent space in a triangular coordinate system (Jacobs, et al. 2013). Many research studies have explored the presence and function of place and grid cells in rodents, bats, and primates. One study showed that rats have a hippocampal cognitive map, representing specific objects in specific locations, and only secondarily focusing on object identity (Manns & Eichenbaum, 2009).
In a recent study, researchers directly obtained electrophysiological recordings from humans while they were undergoing treatment for epilepsy (Jacobs, et al. 2013). The subjects performed a virtual spatial learning task in between clinical procedures. The subjects used a joystick to navigate a virtual environment, and during testing they needed to travel directly from one object to an invisible goal object. Microelectrodes placed in the subject’s brain recorded neural activity throughout the task. The data provide evidence that humans, just like other mammals, have an allocentric spatial cognitive map, complete with grid cells and place cells. This means the location of one object is defined in relation to the location of others.
All of these data help to explain my journey to Montmartre. As I pictured myself walking through the area that I wished to go, my hippocampus fired to simulate the future, goal-directed action (Wikenheiser & Redish, 2015). Just as your place cells fire to represent where you are, they can also fire as you “replay” an experience in your mind. When I found the train platform closed, my interconnected hippocampus likely communicated with the reward center of my brain in the ventral striatum to link the failed expectation to the place (van der Meer, & Redish, 2009). Once I started navigating through the highly unfamiliar area, the neurons in my hippocampus and entorhinal cortex must have fired in an effort to link novel stimuli to spatial context. Evidence suggests that my instinct to “walk north” was because exploration relies heavily on grid-cells with direction sensitivity (Jacobs, et al. 2013). As the scenery around me started to become more familiar, specific cells throughout my hippocampal formation responded to locations, landmarks, and directions that triggered unique firing patterns (Jacobs, et al. 2013). Once I arrived at my final destination, my hippocampal neurons would have fired to help many areas of my brain realize that this restaurant was familiar, and that I should soon expect a reward in the form of delicious French food.
I am looking forward to many more adventures through Paris, and I now know much more about how my hippocampus can dependably orient me in space. I also learned that I probably should start carrying a map.
Jacobs J, Weidemann CT, Miller JF, Solway A, Burke JF, Wei X, Suthana N, Sperling MR, Sharan AD, Fried T, & Kahana MJ, (2013). Direct Recordings of grid-like neuronal activity in human spatial navigation. Nature Neuroscience. 16(9):1188-1190
Manns JR, & Eichenbaum H, (2009). A cognitive map for object memory in the hippocampus. Learning & Memory. 16:616-624.
van der Meer MAA, Redish AD, (2009) Covert expectation-of-reward in rat ventral striatum at decision points. Front Integr Neurosci. 3:1-15.
Wikenheiser AM, & Redish AD, (2015). Decoding the cognitive map: ensemble hippocampal sequences and decision making. Current Opinion in Neurobiology. 32:8-15.