Medical Examiner

What Makes Human Brains Special?

Scientists are just starting to grasp why human beings have self-awareness.

MRI of brain.
DouglasOlivares/iStock/Getty Images Plus

This excerpt is adapted with permission from Know Thyself: The Science of Self-Awareness by Stephen M. Fleming, published by Basic Books. Copyright 2021 by Stephen M. Fleming.

A variety of preserved brains can be seen at the Hunterian Museum in London, near the law courts of Lincoln’s Inn Fields. The museum is home to a marvelous collection of anatomical specimens amassed by John Hunter, a Scottish surgeon and scientist working at the height of the 18th-century Enlightenment. I first visited the Hunterian Museum shortly after starting my Ph.D. in neuroscience at University College London. I was, of course, particularly interested in the brains of all kinds—human and animal—carefully preserved in made-to-measure jars and displayed in rooms surrounding an elegant spiral staircase. All these brains had helped their owners take in their surroundings, seek out food, and (if they were lucky) find themselves a mate. Before they were each immortalized in formaldehyde, these brains’ intricate networks of neurons fired electrical impulses to ensure their host lived to fight another day.

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Each time I visited the Hunterian, I had an eerie feeling when looking at the human brains. On one level, I knew that they were just like any of the other animal brains on display: finely tuned information-processing devices. But it was hard to shake an almost religious feeling of reverence as I peered at the jars. Each and every one of them once knew that they were alive. Now, as a neuroscientist with my own lab, I still wonder: What is it about the human brain that gives us these extra layers of recursion and allows us to begin to know ourselves? What is the magic ingredient? Is there a magic ingredient?

One clue comes from comparing the brains of humans and other animals. It is commonly assumed that humans have particularly large brains for our body size—and this is partly true, but not in the way that you might think. In fact, comparing brain and body size does not tell us much. It would be like concluding that a chip fitted in a laptop computer is more powerful than the same chip fitted into a desktop, just because the laptop has a smaller “body.” This kind of comparison does not tell us much about whether the brains of different species—our own included—are similar or different.

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Instead, the key to properly comparing the brains of different species lies in estimating the number of neurons in what neuroscientist Suzana Herculano-Houzel refers to as “brain soup.” By pureeing the (dead!) brains of lots of different species with detergent, and then counting the number of nuclei that remain, she can plot the actual number of cells in a brain against the brain mass, enabling meaningful comparisons to be made.

After several painstaking studies of brains of all shapes and sizes, a fascinating pattern has begun to emerge. The number of neurons in primate brains (which include monkeys, apes such as chimpanzees, and humans) increases linearly with brain mass. If one monkey brain is twice as large as another, we can expect it to have twice as many neurons. But in rodents (such as rats and mice), the number of neurons increases more slowly and then begins to flatten off, in a relationship known as a power law. This means that to get a rodent brain with 10 times the number of neurons, you need to make it 40 times larger in mass. Rodents are much less efficient than primates at packing neurons into a given brain volume.

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It’s important to put this result in the context of what we know about human evolution. Evolution is a process of branching, rather than a one-way progression from worse to better. We can think of evolution like a tree—we share with other animals a common ancestor toward the roots, but other groups of species branched off the trunk many millions of years ago and then continued to sprout sub-branches, and sub-branches of sub-branches, and so on. This means that humans (Homo sapiens) are not at the “top” of the evolutionary tree—there is no top to speak of—and instead we just occupy one particular branch. It is all the more remarkable, therefore, that the same type of neuronal scaling law seen in rodents is found both in a group that diverged from the primate lineage around 105 million years ago (the afrotherians, which include the African elephant) and a group that diverged much more recently (the artiodactyls, which include pigs and giraffes). Regardless of their position on the tree, it seems that primates are evolutionary outliers—but, relative to other primates, humans are not.

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What seems to pick primates out from the crowd is that they have unusually efficient ways of cramming more neurons into a given brain volume. In other words, although a cow and a chimpanzee might have brains of similar weight, we can expect the chimpanzee to have around twice the number of neurons. And, as our species is the proud owner of the biggest primate brain by mass, this creates an advantage when it comes to sheer number of neurons. The upshot is that what makes our brains special is that (a) we are primates, and (b) we have big heads!

We do not yet know what this means for our unusual capacity for self-awareness. But, very roughly, it is likely that there is simply more processing power devoted to so-called higher-order functions—those that, like self-awareness, go above and beyond the maintenance of critical functions like homeostasis, perception, and action. We now know that there are large swaths of cortex in the human brain that are not easy to define as being sensory or motor, and are instead traditionally labeled as association cortex—a somewhat vague term that refers to the idea that these regions help associate or link up many different inputs and outputs.

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Regardless of the terminology we favor, what is clear is that association cortex is particularly well-developed in the human brain compared with other primates. For instance, if you examined different parts of the human prefrontal cortex (which is part of the association cortex, located toward the front of the brain) under the microscope, you would sometimes find an extra layer of brain cells in the ribbonlike sheet of cortex known as a granular layer. We still don’t fully understand what this additional cell layer is doing, but it provides a useful anatomical landmark with which to compare the brains of different species. The granular portion of the PFC is considerably more folded and enlarged in humans compared with monkeys and does not exist at all in rodents. It is these regions of the association cortex—particularly the PFC—that seem particularly important for human self-awareness.

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Many of the experiments that I run in my laboratory today, along with a whole host of research associates and graduate students, are aimed at understanding how these parts of the human brain support self-awareness. If you were to volunteer your (live!) brain for one of our studies, we would meet you in our colorful reception, decorated with images of different types of scanners at work, and then we would descend to the basement where we have an array of large brain scanners, each arranged in different rooms. After filling in forms to ensure that you are safe to enter the scanning suite—magnetic resonance imaging uses strong magnetic fields, so volunteers must have no metal on them—you would hop onto a scanner bed and see various instructions on a projector screen above your head. While the scanner whirs away, we would ask you a series of questions: Do you remember seeing this word? Which image do you think is brighter? Occasionally, we might also ask you to reflect on your decisions: How confident are you that you got the answer right?

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If while in the scanner I asked you to think about yourself, it’s a safe bet that I would observe changes in activation in two key parts of the association cortex: the medial PFC and the medial parietal cortex (also known as the precuneus), which collectively are sometimes referred to as the cortical midline structures. Robust activation of the medial PFC is seen in experiments where people are asked to judge whether adjectives such as “kind” or “anxious” apply to either themselves or someone famous, such as the British queen. Retrieving memories about ourselves, such as imagining the last time we had a birthday party, also activates the same regions.

Findings such as these are giving us clues as to the neural machinery of self-reflection. But we are still maddeningly far from a full understanding of how this machinery works. Self-awareness is a continuum, rather than an all-or-nothing phenomenon. Many of the psychological building blocks for self-awareness—like tracking uncertainty and monitoring our actions—can operate unconsciously, providing a suite of neural autopilots that are widely shared across the animal kingdom, and are present early in human infancy. Self-awareness continues to crystallize in toddlers, becoming fully formed between the ages of 3 and 4. We don’t yet understand how this happens. But one promising idea is that self-awareness is grounded in algorithms for modeling minds in general—the brain’s ability to picture not just its own functioning, but that of others too. And thanks to tools and scientific ingenuity that allows us to peer inside the skull, we are now in a better position than ever before to begin understanding how those brains in museum jars once knew they were alive.