When you visit a new city for the first time, you have to roam around for a while to find destination. In a new neighborhood, you often find yourself repeatedly checking Google Maps or asking locals, “Hey, which way is this place?” Instead of reaching your destination, you sometimes lose your way! However, your brain is actively trying to map the place correctly. After a few weeks, that place will become familiar to you. You will also learn some shortcuts like the locals, and when new friends come to visit, you’ll confidently show them around.
But how this happened? How this city become so familiar from unknown labyrinth? Your your brain actually created a cognitive map. This map is like a mental counterpart of the external geography, helping you decide which route to take from one place to another.
How does the brain create this mental map of a new place?
Let’s get acquainted with the hippocampus. In the brain, hippocampus helps us navigate from one place to another. When you hear the word “hippocampus,” you might think of a hippopotamus wandering through a university campus. In reality, the name comes from the Greek word for “seahorse.” This part of the brain, located centrally, looks somewhat like a seahorse. In the past century, the hippocampus was the center of research interest for scientists. However, for a long time, the specific role of the hippocampus remained unknown. Now we know that this part of the brain plays a crucial role in memory, finding ways, and planning for future.
In the 1970s, two researchers — John O’Keefe and Jonathan Dostrovsky — recorded electrical signals or spikes from the hippocampus of a rat. They believed that when the rat was in a specific location, certain neurons in the hippocampus would become active, resulting in spikes or electrical signals.
Let’s imagine that experiment. We placed a rat in a small maze and started recording electrical signals from specific neurons in the rat’s hippocampus.
When a spike is detected from a neuron, it means that the neuron is actively exchanging signals with the neuron, to-and-fro nearby neurons. This spike is not just anything; it represents a specific threshold crossing in electro-chemical potential. It’s the fundamental unit of communication between two neurons.
So, when a spike is generated in the rat’s brain as it explores the maze, we will record the coordinates of the rat’s location in the form of spatial markers.
After the rat has roamed around the maze several times, we will observe that this particular neuron in its brain has become a devout follower of a specific location in the maze. This is because whenever the rat goes to that specific spot or around it, the neuron gets excited and starts generating spikes in an electrochemical frenzy.
Based on this research, these neurons are given the names “place cells,” and the specific area in the maze where these place cells become active is called a “place field.” The place field is essentially a specific spot in the external world where the rat is navigating, and the place cell is located within the hippocampus of the brain. However, we cannot say that the place cell and the place field are directly related in terms of size. A place cell can have a larger or smaller place field for a given location.
Now, you might be wondering since we are three-dimensional beings, does place cell exist for vertical axis as well? Do we only activate them when we move from one place to another, or do they have a more active role in our navigation?
To determine this, researchers conducted a fascinating experiment. They trained a rat to run on a treadmill, which is not just any treadmill but a spherical one, meaning the rat can run in any direction, just like a ball on an old computer mouse. Then they devised a mechanism to keep the rat’s head fixed in one place while displaying a virtual reality screen in front of its eyes.
The research is quite amusing because, even though the rat is running aimlessly on the treadmill, it remains stationary in one location. However, in front of its eyes, a completely different world is displayed, where an open space for exploration is shown. The rat’s brain believes that it is wandering in that open space. Since the rat’s brain is gaining the experience of roaming freely in this virtual world of the cylindrical maze, its hippocampus’s place cells also react similarly.
If we focus on the results of this experiment, the following observations stand out. When monitoring different place cells in various place fields on this virtual cylindrical track, it’s evident that one group of neurons becomes active at the beginning of the track, while another group lights up towards the end of the track. Although the rat remains stationary at a single physical location, its brain engages various place cells even in a virtual environment. In other words, these place cells work not only in the real-world environment but also in the virtual reality.
Researchers modified this experiment by training the rat to run on the treadmill, but when it reached the end of the virtual track, it was rewarded with a treat, like food. The question was, if we artificially excite the place cells that become active at the end of the track when the rat hasn’t physically reached that point, what happens? Does the rat get confused? Surprisingly, the rat begins to lick at that specific place even though it never physically reached that location, indicating that the place cell’s activity influences the behavior of the rat, even in a remote location.
These experiments have shown that place cells construct our mental map. Place cells are not a by-product of our exploration; instead, it is these cells that determine where we are on our mental map.
Now, we might consider how place cells, with their different locations in the environment, relate to the physical surroundings. Do place cells operate in the same way for physical places such as home, office, and market? It appears that they do not. The function of place cells, in part, depends on the specific configuration of an environment. If you alter the environment — by changing the color of your room’s walls or moving furniture to a new spot — place cells may be remapped. However, introducing a new picture frame to the wall will not lead to significant remapping; the place cell will mostly remain unchanged. How the environment affects the relationship between place cells and place fields is something scientists are still working to fully understand, but there are intriguing implications to consider.
As you change the color of the walls in your home and rearrange the furniture to a new location, the spatial relationship is altered. However, if you hang a new picture frame on the wall, the spatial configuration might remain the same. The rules governing how spatial relationships and spatial maps change with changes in the environment are not yet fully understood by scientists. Nevertheless, some intriguing discoveries have been made.
For instance, if you place a rodent in a cylindrical chamber and provide it with a card inside the chamber, turning the card will not only change the location of the rodent, but it may also alter the relative spatial relationships between different locations. On the other hand, if you keep the chamber the same size and shape but change its orientation, the spatial relationships remain unchanged. We can deduce from various observations related to spatial mapping that the hippocampus doesn’t just determine the location of an organism within its external environment; it also conveys a message about the environment itself.
Interestingly, even if the rodent in the same environment perceives a change in some feature that’s not directly related to location, it can still lead to spatial remapping. This kind of feature could be a new smell or an electrical shock. If, for example, you condition a rodent in a familiar environment to associate a specific smell with fear by exposing it to a mild electrical shock while near the source of that smell, it will develop a fear of that particular place in its mental map, even if there haven’t been any changes in the local environment.
These remapping observations raise questions about whether it’s appropriate to call these neural structures “spatial maps.” While initially, these structures were linked to the representation of location in space, we now know they can deal with information not directly related to location. Another well-known experiment in this regard involves manipulating the sound control in the environment.
First, the rodent is exposed to a specific sound frequency and needs to explore to obtain a reward (food). Then, it is given a joystick to manipulate the sound frequency in a speaker to control the reward. The rodent receives a reward only when it can generate the target frequency voluntarily. Although the rodent may initially fumble with the joystick, it eventually figures out how to create the required sound frequency. At that point, it receives a reward. The researchers observed that the hippocampus started encoding place fields in response to the sound frequency. When the joystick was used to control the sound frequency, the place fields associated with these different frequencies became active, demonstrating how even in the absence of a change in local environment, place fields can be dynamically remapped in response to voluntary control of the environmental stimulus.
This research adds a new dimension to our understanding of spatial mapping. The spatial map is not just limited to a physical location in an environment but can involve conceptual and perceptual features. While we may not fully understand how these neural mechanisms work in the brain, this research underscores the complexity and adaptability of the brain’s spatial mapping system.
One intriguing question that remains is whether the term “spatial mapping” is still an accurate description for these functions. Although initial research into these structures primarily focused on their role in mapping physical locations within an environment, we now see that they can adapt and respond to other aspects of the environment, expanding our understanding of how the brain processes and represents spatial information.
Source: I heavily used the excellent video documentary on this topic by Artem Kirsanov — Place cells: How your brain creates maps of abstract spaces
- Aronov, D., Nevers, R., Tank, D.W., 2017. Mapping of a non-spatial dimension by the hippocampal–entorhinal circuit. Nature 543, 719–722.
- Christopher D. Harvey, Forrest Collman, Daniel A. Dombeck, David W. Tank, 2009. Intracellular dynamics of hippocampal place cells during virtual navigation. Nature.
- Grieves, R.M., Jedidi-Ayoub, S., Mishchanchuk, K., Liu, A., Renaudineau, S., Jeffery, K.J., 2020. The place-cell representation of volumetric space in rats. Nat Commun 11, 789.
- Jeffery, K.J., 2011. Place Cells, Grid Cells, Attractors, and Remapping. Neural Plasticity 2011, 1–11.
- Latuske, P., Kornienko, O., Kohler, L., Allen, K., 2018. Hippocampal Remapping and Its Entorhinal Origin. Front. Behav. Neurosci. 11, 253. Laura Lee Colgin, Edvard I. Moser, May-Britt Moser, 2008.
- O’Keefe, J., Burgess, N., 1996. Geometric determinants of the place fields of hippocampal neurons. Nature 381, 425–428.
- Robinson, N.T.M., Descamps, L.A.L., Russell, L.E., Buchholz, M.O., Bicknell, B.A., Antonov, G.K., Lau, J.Y.N., Nutbrown, R., Schmidt-Hieber, C., Häusser, M., 2020. Targeted Activation of Hippocampal Place Cells Drives Memory-Guided Spatial Behavior. Cell 183, 1586–1599.e10.
- Wohlgemuth, M.J., Yu, C., Moss, C.F., 2018. 3D Hippocampal Place Field Dynamics in Free-Flying Echolocating Bats. Front. Cell. Neurosci. 12, 270.