Fernanda Pérez Gay Juárez, translated from Spanish by Álvaro García //
“Hippocampus” is the scientific name for the seahorse, an S-shaped fish with ringed, bony plates and a dorsal crest. Its tail is long, prehensile and coiled in spiral, and its head resembles that of a horse. Before reproducing, two seahorses intertwine in an eight-hour dance, an essential part of their mating ritual. Hippocampi are fish of tiny, multiple fins, which flap hastily—fish that swim in an upright position.
In Greek mythology, hippocampi were sea monsters, similar to aquatic horses: with the head and front legs of a horse but the winding tail of a fish or dolphin. Poseidon, God of the Sea, was carried across the oceans in a chariot pulled by hippocampi, who sometimes took him out of the water. He was the man of horses, earthquakes and seas. It is said that a temple to Poseidon was built thousands of years ago on a city on the Greek coasts called Helike. After an earthquake, the city was submerged, and with it the temple of the God of the Sea, surrounded by his loyal, marble hippocampi. For the Phoenicians, the hippocampus held the combination of commerce, represented by the horse, and seafaring, represented by the dolphin. Their coins bore hippocampi: swimming horses, some of them winged.
In neuroscience, the hippocampus is a structure hidden within the temporal lobe of the brain. Humans and other mammals have two hippocampi; two symmetrical curved halves, one in each cerebral hemisphere. Folded into a semi-circular structure, the hippocampal neurons string together past and future. They are an essential part of the brain apparatus that consolidates new memories and weighs possible outcomes. In the sixteenth century, when anatomist Giulio Cesare Aranzio observed these horns located alongside the ventricles of the brain, he was unsure whether to call them “hippocampi” or “silk worms.”1 He decided on the first option, conceding their resemblance to the seahorse, or perhaps inspired by Greco-Roman mythology.
After hours of analyzing figures that reflect patterns of electrical activity in the brain, I am struck by the subtitle of an article on the history of the brain’s anatomy in a neuroscientific journal: “From poetics to statistics: evolving from visual inspection and verbal descriptions to observer independent metrics.”2 This is a reference to the way in which our acquisition and description of knowledge has changed: Before technology advanced enough to have objective and quantitative measurements of the nervous systems, anatomy was the main approach to the brain. The early neuroanatomists were on the verge of art and science, relying on drawings and poetic descriptions of the specimens they worked on throughout their studies. Today, living in the boom of neuroimaging and powerful programs of statistical analysis, I am sometimes envious of those men who, prying into the morphology of the brain, saw seahorses on its surface.
In the 1950s, the acclaimed surgeon William Scoville operated on a man with intractable epilepsy and removed part of his temporal lobes, along with his hippocampi. The man’s epilepsy was cured, but from that moment on, he was unable to set in new memories. The initials of that man are HM —indelible letters in the history of neuroscience. When neuropsychologist Brenda Milner later performed tests and memory exercises on this patient, she discovered that memory systems rely deeply on the temporal lobes, which hold the hippocampi in their depths.3,4 Brenda Milner, who turned 100-years old last year, is still active at the Montreal Neurological Institute. I have come across her a couple times: There she goes, I think, legendary and smiling, her back mildly arched by age resembling the ringed curvature of a seahorse’s tail.
As part of my work with Brain Reach, a McGill University initiative to disseminate science, once a week I talk to ten-year-old children about the brain. They are always excited to learn that the formation of new memories occurs in the hippocampus, that seahorse-resembling brain structure. I tell them that this part of the brain is fundamental to the consolidation of new memories, and also to spatial navigation. 5 It grants us memory, but also speculation of the future, where to go next in all possible decisions. Children rarely remember the precise names of the parts of the brain that I refer to—they often forget, for instance, the correct pronunciation of the “occipital” lobe. But they never forget the hippocampus.
I like to think that the C buried under the cerebral cortex is full of neurons that weave memories, encrypted in electrical impulses, at the same time I think of the smile of Brenda Milner, to whom we owe the understanding of a great part of its function. I remember that, in 2014, another inspiring woman scientist, May Britt Moser, won the Nobel prize for discovering the so-called grid-cells, the neurons that inform an animal’s hippocampus about where its body is located in space.6 Thanks to these cells, which create a map of the outside world, the hippocampus can store and retrieve movement in space and show laboratory animals the way out of labyrinths. In fact, Moser suggests remembering where we are in space may be the basis of all memory: where we are and where we come from. But I then drift to the idea of the hippocampus as the mythological horse on which the Phoenicians sailed and traded, or as the sea monster on which Poseidon rose out of the sea—his labyrinth. I cannot help it: I love to think about the ritual dance of the hippocampus, the courtship display of that strange S-shaped fish, its ringed, bony plates and its many, tiny fins, beating dizzily.
Contrary to the subtitle of that article, thinking of Brenda Milner and the hippocampus disengages me from statistics and brings me back to poetry: memory —I believe—is a winged horse; a fish swimming in an upright position, a seahorse flying deep into the ocean of our brains.
Fernanda Pérez-Gay Juarez is a medical doctor, born in Mexico City in 1988. She obtained a PhD in Neuroscience at McGill University, working on categorization and visual perception. Her scientific work has been presented in local and international conferences and published in peer-reviewed journals. She is the author of more than 30 science broadcasting articles in English and Spanish, and the head of the project SINAPSIS, Conexiones entre el Arte y tu Cerebro, which shares and explains the links between neuroscience and art for a general audience.
Featured Image: “Neptune’s Horses,” Walter Crane, Public domain, via Wikimedia Commons.
References
1. Engelhardt, E. Hippocampus discovery First steps. Dement. Neuropsychol. 10, 58–62 (2016).
2. Weiner, K. S. & Zilles, K. The anatomical and functional specialization of the fusiform gyrus. Neuropsychologia 83, 48–62 (2016).
3. MILNER, B. Psychological defects produced by temporal lobe excision. Res. Publ. Assoc. Res. Nerv. Ment. Dis. 36, 244–57 (1958).
4. Squire, L. R. The legacy of patient H.M. for neuroscience. Neuron 61, 6–9 (2009).
5. Eichenbaum, H. The role of the hippocampus in navigation is memory. J. Neurophysiol. 117, 1785–1796 (2017).
6. Moser, E. I. et al. Grid cells and cortical representation. Nat. Rev. Neurosci. 15, 466–481 (2014).
