Know Your Brain: The Neocortex — The Light Upstairs

(Editor’s note: This article is from the Spring 2017 issue of Brain World magazineIf you enjoy this article, consider a print or digital subscription!)


IN THE BEGINNING

Lucy, the small primate who once roamed the valleys of Tanzania some 3 million years ago, was our remote ancestor — but chances are she spent much of her life in trees, with limbs adept for leaping across branches. It was only as we began to leave the trees that our brains grew larger; increasingly becoming more equipped to solve problems, to build tools, and to think the deep thoughts we’ve become accustomed to today — cognitive attributes we owe to our brain’s neocortex.

As monumental as conquering the grasslands for the first time probably was, it may have largely been a typo in our own genetic code that made it possible. According to research just unveiled this year at the Max Planck Institute of Molecular Cell Biology and Genetics, in Germany, a point mutation occurred in our brains, paving the way for the existence of the neocortex, one of the most recent evolutionary developments in mammal brains.

This same mutation has been found in prehistoric humans that our own ancestors roamed the Earth with — that is, the Neanderthals and Denisovans, whom you might think of as our evolutionary cousins. Yet, the chimpanzees, our closest surviving relatives, whose genome is 96 percent similar to our own, don’t share this mutation with us — one that would allow stem cells to expand into neurons, thus growing our brains larger, into the ones we know today.

The simple reason for this decisive variation could be one of the minimal differences in our DNA, occurring as our genes mutated. Researchers replaced a single nucleotide — or genetic building block in the gene — changing the C (base cytosine) to G (guanine), and were able to stop the stem cells from developing. And as we began to develop reasoning and language, evolution would set us even further apart from the chimps — as these advancements allowed us to interact with each other and expand our brains even further, as our brains needed to grow in order to support the size of the neocortex region. Today, our ratio for neocortical gray matter to brain size is 60–1, compared to 30–1 for chimps.

While Lucy had not abandoned tree life entirely, newly uncovered evidence suggests that her species used tools and hunted — two advanced skills for primates. She and her family lived in perilous times, when cooperation among the tribe would be rewarded with gathering enough food for survival, and competition, as well as constant attacks from predators, rewarded abilities like swift thinking and spatial reasoning — a process of natural selection that encouraged the neocortex to continue growing.

EXPANDING OUR MINDS

The neocortex is made up of the brain’s gray matter connected by unmyelinated fibers — one of the major components of our central nervous system, consisting of sensory and motor neurons, and located in the cerebrum of the brain. It represents the largest portion of the cerebral cortex, covering both of the brain’s hemispheres. In fact, about 76 percent of our brain is included in the neocortex. The structure is then broken up into the brain’s frontal, parietal, occipital, and temporal lobes, divided by cranial sutures. It is the frontal-lobe portion of the neocortex that makes us unique — an area responsible for language processing, a region of the brain known as Broca’s area. Unlike other mammals — the only group we share this complicated structure with — we’re the only species to have a neocortex made up of six horizontal layers, consisting of cortical columns that organize the neurons. The orbitofrontal cortex is involved in social and emotional processing — accounting for how we learn to act from those around us.

As we grow and acquire new skills, our neocortex will also establish semantic memories — the steps to riding a bicycle or driving a car, complicated tasks that we never really seem to forget as we age. This is a primary function of the brain’s temporal lobe. By turn, the neocortex is also responsible for operant conditioning — as it consists of both the primary visual and primary auditory cortex of the brain; the reason the smell of fresh-baked bread seems to immediately make you hungry.

The neocortex even remains active throughout periods of slow-wave sleep — which is affected by the neurons it fires into the brain. This is perhaps the best state of sleep to be in the night before an exam — critical to the consolidation of memory, something neuroscientists have called sleep-dependent memory processing. As we sleep, the neurons transition between a state of rest and a depolarizing phase known as an upstate. The neocortex’s functioning is one of the primary reasons that a good night’s sleep is so ideal.

Approximately 80 percent of the neocortex neurons are excitatory and about 20 percent inhibitory, with their functionary names derived from how they interact with other neurons. While the importance of the neocortex is well understood, many of its functions are still being explored, particularly the creation of semantic memory.

Unlike episodic memory (when you recall important events in your life: the first drive you took in your first car, or your wedding day, events which almost entirely consist of timed, visual elements), semantic memory acts more like a vast database, in which the neurons transfer information from all six layers of the neocortex — spatial recognition from the primary visual cortex for driving, as well as the auditory information for recalling sounds, for example. Spatial reasoning also comes into account, as you remember the one-thousandth rule before making a turn. The neurons communicate by moving throughout the brain in spindle waves.

The latest research suggests that the temporal lobe acts as a sort of convergence zone where these memories develop — a region of the brain that is also particularly vulnerable to disorders like Alzheimer’s, but also specific types of memory loss, such as semantic dementia, a condition causing the patient to forget the meanings of words. The same mutations that made it possible for the neocortex to expand, so rapidly, have also given us a number of disadvantages. This same gene that we shared with the Neanderthals may have also made us vulnerable to neuropsychiatric disorders like schizophrenia. For these reasons, researchers have paid particular attention to the neocortex as advances in neuroscience continue to develop — leading many researchers to work at building their own brain.

THE BLUE BRAIN PROJECT

In recent years, a Pan-European initiative has gone underway to reconstruct a virtual map of the human brain, known as the Blue Brain Project, with the primary goal of recreating the neocortical column, what neuroscientists currently believe is the smallest unit making up the neocortex. Since the initiative was launched in December 2006 by the Ecole Polytechnique Federale de Lausanne of Switzerland, they not only were able to recreate the column, but are currently pioneering other ways crucial to the advancement of medicine, such as accurately demonstrating the effect calcium bonds have on memory.


The Blue Brain Project has used computer tools to create dendrites — the large branchlike extensions of neurons, which interact in a virtual environment. One recent effort in collaboration with the U.S.-based Allen Institute for Brain Science has been to produce accurate models of neurons, and to anticipate what electronic interactions between them look like. This would make it easier than ever for neuroscientists to make predictions and get inside the heads of their patients, meaning that in the near future, not only will our understanding of the neocortex greatly improve, we’ll also know more of what to expect when looking at functional MRI scans.

(Editor’s note: This article is from the Spring 2017 issue of Brain World magazineIf you enjoy this article, consider a print or digital subscription!)

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