Deep within the brain, on the medial shore of the temporal lobe, sits the amygdala, a venerable old lighthouse. Its history reaches back into the misty dawn of the vertebrate orders, long before the great cortical towers were erected. Legends suggest that the amygdalar keep was one of the earliest settlements built after the great neuronal migration. Not far from the amygdala are shards of pottery suggesting urheimats — the birthplaces of pioneer neurons which, over the architectonic timescales, became the founders of cortical empires.
Two amygdalar chambers can be traced by the anatomist. One chamber, the basolateral, houses corpulent pyramidal neurons, cousins of the celebrated pyramids that dominate the cortical landscape. Another chamber, the central, houses spiny projection neurons, which show markings of kinship with residents of the striatum — the ancient market town where, to this day, transactions are mediated in the currency of dopamine. With one chamber resembling the cortical manufactories and another the striatal emporia, the amygdala seems a telencephalon in miniature. A premonition.
But these two chambers are not the only rooms in the amygdala. A slender hallway both divides and joins them; a band of cells know as the intercalated masses. To this day, some brain atlases simply ignore these borderline neurons. Perhaps things that help mark boundaries are confused with the boundaries themselves.
I first came in contact with the sprawling (and still-incomplete) saga of brain evolution when I joined my current lab, right after grad school. It’s a neuroanatomy lab, carrying on the grand tradition of Santiago Ramón y Cajal, the father of neuroscience. But in stark contrast to my collaborators, I’m a computational modeler — far removed from lab coats, protocols, rubber gloves, pipettes, microscopes. I use code and equation as an artist does canvas and pigment — to create impressionist portraits of neural phenomena. The kind of modeling I do remains a somewhat niche pursuit in neuroscience, precisely because of its impressionistic quality. The task of modeling is ostensibly to draw lines connecting the dots that are experimental findings — and perhaps anticipate a few dots before they are produced. But the dots are innumerable, and every other day someone from Stanford seems to develop a new way to generate more of them.
My first project in the lab was to help make sense of the intercalated masses — the neurons of the amygdalar gaps. They are a small and not-particularly-famous group of neurons; few would even think to correlate their chatter with grand ponderables like ‘cognition’, ‘empathy’, or ‘consciousness’. But they receive targeted input from brain areas that do get associated with those words — crucial parts of the prefrontal cortex. My little model of the intercalated masses may well contribute to the multigenerational quest to understand emotion and motivation, and to one day tame mental illness. This is of course unlikely, given the deluge of data-dots.
But even if the world doesn’t learn much from my model, I may have learned a thing or two while developing it. For instance, I could see very clearly, even with my broad brushstrokes, that one cannot judge the influence of something by its size or its apparently marginal position. The tiny intercalated masses (or at the very least, my simulation-paintings of them) appear to be a pivot of the emotional system, enabling swings from caution to recklessness. From another perspective, the neurons in this liminal zone are functionaries of the cortex, manipulating ‘facts on the ground’ to promote — or to suppress — emotional tumult.
The word ‘intercalated’ is related to the word ‘intercalary’. An intercalary day (or week, or month) is inserted into a calendar to keep it in sync with larger cosmic events. A leap day is a classic example. It is inserted once every four years to keep our otherwise-365-day calendars aligned with the solar year, which is closer to 365-and-a-quarter days long.
Intercalation is a reminder that rational constructions are simultaneously useful and incomplete. In antiquity, rationality was about ratio. The cosmos tantalized the early numerate humans with the possibility of whole number proportions: almost 30 days in a month (or ‘moonth’), almost 12 moons in a year, almost 13 cycles of Venus in 8 years… the list goes on. The ‘perfect’ versions of some of these almost-ratios are, coincidentally, used to tune musical instruments. But the key word here is almost. Without intercalation, our ‘rational’ cycles would fall out of harmony with the heavenly spheres. It is as if the gods are jazz musicians, and mortals are simply trying to keep up with their intricate — yet strangely loose — sense of rhythm.
So intercalation is a name for the filling of holes in our understanding. It is the reasonable amendment of our rational models, in the interest of coherence with the world. It is how we can take unforeseen circumstances in stride; we mind the gaps without getting tripped up.
My life is a series of interwoven attempts at intercalation. Disciplinary boundary-crossings. Silk routes through intellectual hinterlands. Marginalia. My quest is to mine these gaps — and I confess, I do occasionally trip on them.
Notes
More on the Mayan calendar here: The Classic Maya Calendar and Day Numbering System
I find myself frequently revisiting the idea of ‘jazz cosmology’. Here’s the latest iteration: Sailing the Seas of Thought
Image sources
Janak, Patricia H., and Kay M. Tye. "From circuits to behaviour in the amygdala." Nature 517, no. 7534 (2015): 284-292.
Zikopoulos, Basilis, Yohan J. John, Miguel Ángel García-Cabezas, Jamie G. Bunce, and Helen Barbas. "The intercalated nuclear complex of the primate amygdala." Neuroscience 330 (2016): 267-290.
"One chamber, the basolateral, houses corpulent pyramidal neurons, cousins of the celebrated pyramids that dominate the cortical landscape. Another chamber, the central, houses spiny projection neurons, which show markings of kinship with residents of the striatum — the ancient market town where, to this day, transactions are mediated in the currency of dopamine."
I could read a whole book of such prose .. just sayin :P
The bit about calendars / keeping in sync reminds me of this weird book review I read recently , thought you may find interesting
https://www.sbnation.com/secret-base/21514470/yoon-ha-lee-machineries-of-empire-books