THE HIPPOCAMPUS HAS A RELEVANT ROLE FOR OUR MEMORY, ESPECIALLY WITH THE EPISODE, BUT ITS FUNCTION GOES MUCH BEYOND. THIS STRUCTURE CATEGORIZES OUR REALITY, REACTING TO THE INFORMATION IT RECEIVES.
Inside the temporal lobes, in the depths of our brain, there is a structure that is shaped like a curved tube, just a few centimeters long, and in which the 16th century neuroanatomist Giulio Cesare Arancio wanted to see something similar to a seahorse, especially when considered together with the fornix, a set of fibers that emerge from it and join both hemispheres. Hence the name hippocampus, the Greek name for this small animal and by which this structure is known, in which they also wanted to see the shape of a small ram’s horn or, to be creative, of the Egyptian god Amun. For this reason, one of its subdivisions is known as Ammon’s horn.
It only has three layers of neurons, compared to other more evolved areas of the cerebral cortex, such as the neocortex, which typically has six layers. The hippocampus is therefore part of what is known as the archicortex, or “ancient cortex” in Greek, a structure that traces its origins to about 450 million years ago, with the first vertebrates.
Due to its “primitivism”, we might think that the hippocampus must be a structure of little importance, especially in the human brain, where the neocortex plays a very prominent role and whose increase in extension has been one of the most main and defining features of our evolutionary trajectory. But nothing is further from this. The hippocampus has been giving us surprises for some time now.
Century. The role of the hippocampus was still unclear. It could participate in emotions, given its connections with other structures, or perhaps also in smell; but little else could be stated. Hence, when the young American Henry Molaison was diagnosed with severe and recurrent temporal lobe epilepsy, neurosurgeons had no major qualms about removing large parts of his temporal lobes, including both hippocampi. The result could not be more dramatic: Molaison went down in history as “patient HM” (his full name was revealed after his death in 2008), considered the case that marked a before and after in our knowledge about the hippocampus. From knowing hardly anything to giving it a fundamental role for our memory, especially for episodic memory, which has to do with what we remember about events. Patient HM remembered his past life, but after the operation he was unable to store new memories. Nothing at all, except what had to do with motor skills. He didn’t even remember that he had had surgery and that he had this problem, known as anterograde amnesia, so when he got up every day he expected to see in the mirror that young man who had surgery at the age of 27. Imagine the daily shock he must have suffered until his death, at 82.
That the role of the hippocampus in memory turned out to be so important, while this function had not been discovered in animal studies, led to much speculation. Thus, it was said that only in humans the function of this structure is to store memories, but not in other living beings. It was also believed that memories were stored in the hippocampus itself, which would be something like our “hard drive.” Over time, the function of the hippocampus in memory was recognized in many other mammals, along with another of its main functions: spatial orientation or, more specifically, the memorization of sequences of movements or places in space. Its role as a storehouse of memories also became more nuanced over the years: from being the permanent depository it became only the temporary one, since after a time the memories would be transferred to the neocortex. Its role in memory became increasingly clearer, until it was concluded that its main action is actually to review the neocortical circuits that constitute our memories until they are consolidated, which occurs because these circuits undergo physical transformations that facilitate their activation. I always like to surprise people by saying that these reviews that the hippocampus does of our memories in our neocortex can take up to three years, that it does so without us being aware of it, and that most likely now, you and I, we are reviewing dozens, hundreds of memories, without knowing it.
The last few years have brought even more surprises, and most likely within a decade we will have completely reconfigured our knowledge of it. Thus, the role of the hippocampus goes far beyond memory. Or not, but we would have to understand memory as something completely dynamic. It is a trend in neuroscience right now that the brain is continually anticipating what is going to come at every moment, it is predicting how things are; He anticipates, prepares how and what he is going to perceive. Perceiving would therefore not be a merely receptive, passive process, but a complex act in which the brain proposes and the sense organs – both external and from our own body, including our viscera – confirm or deny, in a process according to which we are categorizing at each moment. That is, we are giving meaning to what we perceive: «this is this, or that; such a thing is happening…” Even our emotions would be produced like this. Not in vain, the perceptual areas of the brain receive ten times more connections from within the brain itself than from the sense organs. That’s memory.
And, well, some authors are realizing that the information that reaches the brain ultimately ends up in primitive regions of the brain, among which the hippocampus stands out, and that from there signals arise that also ultimately end up in the perceptual and motor regions of the brain, in a categorization loop, where the information is increasingly abstract and integrative the closer we are to the archicortex of which the hippocampus is a part. If the reader remembers what I said in these pages about the default network, it will not be strange to understand that the hippocampus would be part of it. Some authors go further, and in these categorization functions give the hippocampus and other ancient structures a fundamental role in the regulation of the visceral and immune systems, establishing a non-negligible connection with our mental and physical health. The future of the hippocampus in the clinic is already here.
It has just been published in one of the most prestigious scientific journals that the hippocampus reacts to very complex information of all kinds, even when we are anesthetized and totally unconscious. The “seahorse” anticipates the type of sounds that are going to come in a sequence and reacts if it breaks, and is capable of responding to complex linguistic stimuli, classifying words, reacting to their meanings and whether or not they fit at each moment. These are functions that until now were thought to require us to be aware of them. It doesn’t surprise me, really, because as I mentioned recently, AI can do many of these things without having an iota of consciousness. What is perhaps more surprising is that the hippocampus participates in these functions, undoubtedly the result of what it receives from other parts of the brain, in its integrative and abstract role necessary to categorize reality. We remain expectant as to what becomes known about this structure that was once so unknown, but whose prominence seems to be increasing.
