Orientation and exploration in new or unknown spaces is one of the cognitive faculties that we use most often. We use it to orient ourselves in our house, our neighborhood, to go to work.
We also depend on it when we travel to a new and unknown city for us. We use it even when we drive and, possibly, the reader on some occasion has been the victim of carelessness in his or her orientation or that of a colleague, which will have condemned him to getting lost, being forced to drive around in circles until he crashes. with the proper route.
It’s not the fault of the orientation, it’s the fault of the hippocampus
All of these are situations that tend to frustrate us a lot and that lead us to curse our orientation or that of others with insults, screams and various behaviors. Good, Well, today I will give a glimpse into the neurophysiological mechanisms of orientation in our Brain GPS to understand us.
We’ll start by being specific: we shouldn’t curse orientation since it is just a product of our neural activity in specific regions. Therefore, we will start by cursing our hippocampus.
The hippocampus as a brain structure
Evolutionarily, the hippocampus is an ancient structure, it is part of the archicortex, that is, those structures that are phylogenetically older in our species. Anatomically, it is part of the limbic system, in which other structures such as the amygdala are also found. The Limbic System is considered the morphological substrate of memory, emotions, learning and motivation.
The reader, possibly if he is used to psychology, will know that the hippocampus is a necessary structure for the consolidation of declarative memories, that is, with those memories with episodic content about our experiences or semantic ones (Nadel and O’Keefe, 1972). .
Proof of this are the abundant studies that exist about the popular case of “patient HM”, a patient who had had both temporal hemispheres removed, producing devastating anterograde amnesia, that is, he could not memorize new facts although he retained most of them. of his memories from before the operation. For those who want to delve deeper into this case, I recommend the studies by Scoville and Millner (1957) who exhaustively studied patient HM.
Place Cells: what are they?
Up to this point we are not saying anything new, nor anything surprising. But it was in 1971 when by chance a fact would be discovered that led to the beginning of the study of navigation systems in the brain. O’keefe and John Dostrovsky, using intracranial electrodes, were able to record the activity of specific neurons in the hippocampus in rats This offered the possibility that while they performed different behavioral tests, the animal was awake, conscious and moving freely.
What they did not expect to discover was that there were neurons that responded selectively depending on the area in which the rat was. It is not that there were specific neurons for each position (there is no neuron for your bathroom, for example), but that cells that marked reference points that could adapt to different spaces were observed in CA1 (a specific region of the hippocampus). .
These cells were called place cells. Therefore, it is not that there is a place neuron for each specific space that you frequent, but rather they are reference points that relate you to your environment; This is how egocentric navigation systems are formed. Place neurons will also form allocentric navigation systems that relate elements of space to each other.
This discovery perplexed many neuroscientists, who considered the hippocampus as a declarative learning structure and now saw how it was capable of encoding spatial information. This gave rise to the “cognitive map” hypothesis, which would postulate that a representation of our environment would be generated in the hippocampus.
Just as the brain is an excellent map generator for other sensory modalities such as the encoding of visual, auditory and somatosensory signals; It is not unreasonable to think of the hippocampus as a structure that generates maps of our environment and guarantees our orientation in them
Research has gone further and has tested this paradigm in very diverse situations. It has been seen, for example, that place cells in maze tasks fire when the animal makes errors or when it is in a position in which the neuron would normally fire (O’keefe and Speakman, 1987). In tasks in which the animal must move through different spaces, it has been seen that place neurons fire depending on where the animal comes from and where it is going (Frank et al., 2000).
How spatial maps are formed
Another of the main focuses of research interest in this area has been on how these spatial maps are formed. On the one hand, we could think that place cells establish their function based on the experience we receive when we explore an environment, or we could think that it is an underlying component of our brain circuits, that is, innate. The issue is still not clear and we can find empirical evidence that supports both hypotheses.
On the one hand, the experiments of Monaco and Abbott (2014), which recorded the activity of a large number of place cells, have seen that when an animal is placed in a new environment, several minutes pass until these cells begin to fire rapidly. normal. So that, place maps would be expressed, in some way, from the moment an animal enters a new environment but experience would make these maps modified in the future.
Therefore, we could think that brain plasticity is playing a role in the formation of spatial maps. Therefore, if plasticity really played a role, we would expect that mice knockout of the NMDA receptor for the neurotransmitter glutamate – that is, mice which do not express this receptor – would not generate spatial maps because this receptor plays a fundamental role in brain plasticity and learning.
Plasticity plays an important role in maintaining spatial maps
However, this is not the case, and it has been seen that NMDA receptor knockout mice or mice that have been pharmacologically treated to block this receptor express similar patterns of place cell response in new or familiar environments. Which suggests that the expression of spatial maps is independent of brain plasticity (Kentrol et al., 1998). These results would support the hypotheses that navigation systems are independent of learning.
Despite everything, using logic, the mechanisms of brain plasticity must be clearly necessary for the stability in memory of the newly formed maps. And, if that were not the case, what use would be the experience that one gains from walking the streets of their city? Wouldn’t we always have the feeling that it is the first time we enter our house? I believe that, as on so many other occasions, the hypotheses are more complementary than they seem and, somehow, despite the innate functioning of these functions, plasticity must play a role in maintaining these spatial maps in memory
It is quite abstract to talk about place cells and possibly more than one reader has been surprised that the same brain area that generates memories serves as our GPS, so to speak. But we’re not done and the best is yet to come. Now let’s really curl things up. Initially, it was thought that spatial navigation would depend exclusively on the hippocampus when it was seen that adjacent structures such as the entorhinal cortex showed very weak activation depending on space (Frank et al., 2000).
However, in these studies the activity was recorded in ventral areas of the entorhinal cortex and in later studies dorsal areas which have a greater number of connections to the hippocampus were recorded (Fyhn et al., 2004). So that Many cells in this region were observed to fire depending on position, similar to the hippocampus Up to this point, these are results that were expected to be found, but when they decided to increase the area that they would record in the entorhinal cortex, they had a surprise: among the groups of neurons that were activated depending on the space occupied by the animal, there were apparently silent areas – that is, they were not activated–. When the regions that did show activation were virtually joined, patterns in the form of hexagons or triangles were observed. They called these neurons in the entorhinal cortex “network cells.”
By discovering network cells, there was a possibility of solving the question of how place cells are formed. Since place cells have numerous network cell connections, it is not unreasonable to think that they are formed from these. However, once again, things are not so simple and experimental evidence has not confirmed this hypothesis. The geometric patterns that form the network cells have also not yet been able to be interpreted.
Navigation systems are not limited to the hippocampus
The complexity does not end here. Even less when it has been seen that navigation systems are not reduced to the hippocampus. This has expanded the limits of research to other brain areas, thus discovering other types of cells related to place cells: direction cells and edge cells
The direction cells would encode the direction in which the subject moves and would be located in the dorsal tegmental nucleus of the brain stem. On the other hand, border cells are cells that would increase their firing rate as the subject approached the limits of a given space and we can find them in the subiculum – a specific region of the hippocampus. We are going to offer a simplified example in which we will try to summarize the function of each type of cell:
Imagine that you are in the dining room of your house and that you want to go to the kitchen. Since you are in the dining room of your house, you will have a location cell that will fire while you remain in the dining room, but since you want to go to the kitchen you will also have another location cell activated that represents the kitchen. The activation will be clear because your house is a space that you know perfectly and we can detect the activation in both the place cells and the network cells.
Now, start walking towards the kitchen. There will be a group of specific direction cells that will now be firing and will not change as long as you maintain a specific direction. Now, imagine that to go to the kitchen you have to turn right and cross a narrow hallway. The moment you turn, your steering cells will know and another set of steering cells will register the direction you have now taken activating, and the previous ones will deactivate.
Also imagine that the hallway is narrow and any wrong move can cause you to crash into the wall, so your edge cells will increase their firing rate. The closer you get to the hallway wall, the higher the firing rate of its edge cells. Think of edge cells like the sensors some new cars have that make an audible signal when you’re maneuvering into a park. border cells They work in a similar way to these sensors, the closer you are to crashing the more noise they make When you get to the kitchen, your place cells will have told you that you have arrived satisfactorily and since it is a larger environment, your edge cells will relax.
Let’s just complicate everything
It is curious to think that our brain has ways of knowing our position. But a question remains: How do we reconcile declarative memory with spatial navigation in the hippocampus? That is, how do our memories influence these maps? Or could it be that our memories are formed from these maps? To try to answer this question we must think a little further. Other studies have pointed out that the same cells that encode space, which we have already talked about, also encode time Thus, there has been talk of the time cells (Eichenbaum, 2014) which would encode the perception of time.
The surprising thing about the case is that There is increasing evidence supporting the idea that place cells are the same as time cells Then, the same neuron, through the same electrical impulses, is capable of encoding space and time. The relationship of the encoding of time and space in action potentials themselves and their importance in memory remain a mystery.
In conclusion: my personal opinion
My opinion on this? Taking off my scientist’s coat, I can say that Human beings tend to think of the easy option and we like to think that the brain speaks the same language as we do The problem is that the brain offers us a simplified version of reality that it itself processes. In a way similar to the shadows in Plato’s cave. Thus, just as in quantum physics barriers to what we understand as reality are broken, in neuroscience we discover that in the brain things are different from the world that we consciously perceive and we must have a very open mind to the fact that things have no why be how we really perceive them.
The only thing that is clear to me is something that Antonio Damasio usually repeats a lot in his books: the brain is a great map generator Perhaps the brain interprets time and space in the same way to form maps of our memories. And if it seems chimerical to you, think that Einstein, in his theory of relativity, one of the theories he postulated was that time could not be understood without space, and vice versa. Unraveling these mysteries is undoubtedly a challenge, even more so when they are difficult aspects to study in animals.
However, no efforts should be spared in these matters. Firstly out of curiosity. If we study the expansion of the universe or recently recorded gravitational waves, why wouldn’t we study how our brain interprets time and space? And, secondly, many neurodegenerative pathologies such as Alzheimer’s disease have spatio-temporal disorientation as their first symptoms. By knowing the neurophysiological mechanisms of this coding, we could discover new aspects that would help to better understand the pathological course of these diseases and, who knows, whether to discover new pharmacological or non-pharmacological targets.
