The Most Complex Object in the Universe
As neuroscientists never tire of pointing out, the adult human brain is the single most complex object in the universe, and one of the least understood. On average, it weighs about three and a half pounds, most of which is fragile, malleable tissue—so fragile that the brain is the most difficult part of the body to access, remove, handle, and study. A Soviet neuroscientist once likened its consistency to the insides of a watermelon, but even that overstates its structural integrity. If placed on a table, a fresh brain will quickly surrender to gravity and collapse into a heap (more like gelatin than watermelon). Within eight hours, it will begin to decompose, the first part of a dead body to do so. There may indeed be nothing so complex in the universe, nor anything quite as delicate.
The familiar shape of the human brain is somewhat misleading. As a ubiquitous graphic symbol, its most prominent feature, the massive, fissured cerebrum, has come to symbolize the unlimited potential of human thought, if not the very means of man's dominion over the planet. Yet it also bears an unmistakable resemblance to a comical turban, and for most of recorded history it was treated that way.
Until the 1600s, anatomists drew the brain's tortuous surface as a mass of undifferentiated folds, which they likened in their randomness to the folds of the small intestine.1 After puzzling over its purpose, they concluded that the folds were nothing more than an apparatus for the manufacture of phlegm, which the brain squeezed out through the sinuses, and for producing tears, which it squeezed out through the eyes. Only in the last 150 years have scientists come to appreciate what really goes on in those folds, and that their rapid evolution, seemingly accomplished over the last million years, is easily the most impressive achievement in Darwin's universe. In hindsight, the human brain is a triumph of adaptation, so impressive both in size and reputation that until recently it has succeeded in hiding what has in common with the brains of all mammals, which turns out to be quite a bit.
The principal parts of the mammalian brain are the brain stem, the cerebellum, and the forebrain. The stem houses the physical plant. It monitors and regulates unconscious physical processes such as breathing, blood flow, digestion, and glandular secretion. It consists of the medulla, an extension of the spinal cord, a nodule called the pons, and a short connector called the midbrain. The cerebellum, or little brain, lies behind this assembly, and it is aptly named. With its striated exterior and dual hemispheres (at least in primates), it hangs behind the cantilevered back porch of the forebrain like a wasp's nest. Although its role is still not completely understood, the cerebellum is believed to act as a kind of automatic pilot for fine muscle control. If recent studies are correct, it also plays a role in short-term memory, attention, impulse control, emotion, cognition, and future planning. Researchers suspect that it might be a kind of backup unit, an auxiliary brain. Its loss, while far from desirable, is not fatal. The rest of the brain seems to be able to compensate. The forebrain, on the other hand, is indispensable. It is what makes humans human, and, as a result, the search for the anatomical locus of genius, criminality, or insanity begins there.
Neurologists tend to be of two minds about the forebrain. Some see it as two complementary but sometimes competing hemispheres, an uneasy coalition of rationality and impulse. Others attribute the same inner struggle to a cold brain and a hot brain, the entire cerebrum being the source of cool calculation, and a set of nested organs called the limbic system giving rise to hot instincts and urges. The left brain-right brain dichotomy originated in the 1960s when neurosurgeons intervened in acute cases of epilepsy by severing the corpus callosum, the fiber bundle that allows the two hemispheres to communicate with each other. In most cases the seizures went away, leaving patients with a curious split personality. The notion of a hot brain and a cold brain is somewhat older, and reflects a belief that higher functions, specifically the intellect, are situated literally and figuratively above the lower functions. Just as the intellect is supposed to keep the passions in check, the massive cerebrum envelops the limbic organs—the thalamus, hypothalamus, hippocampus, and amygdala—and, on good days, dominates them. Some psychologists like to refer to the embedded limbic system as the reptile brain, a term they invented as a way to market themselves to Madison Avenue and Hollywood. The impulsive animal brain, they say, seeks dominance, safety, or sustenance, and it wants everything NOW—everything from an ice cream sundae to a sport utility vehicle, with little concern for practicality or consequence. Without the intervention of the cold, rational brain, the reptile brain can act quite unreasonably in getting what it craves.
Whether we really are of two minds in a literal sense is far from proven. Yet there is no denying that every mind undergoes a constant struggle between reason and emotion, between impulse and hesitation, between short-term strategies and long-term planning. The conscious brain struggles to rein in the unconscious, to calm nameless fears and anxieties. But if human beings, which is to say the human brain, hot or cold, can be characterized by one driving force, it would have to be curiosity, which has allowed it to explain just about everything in the universe, with two notable exceptions—the universe and itself.
What distinguishes a human brain from an animal brain, from an actual reptile brain? The size of the cerebrum, for one thing, and thus its surface area. But size, it turns out, isn't everything. The brain of an elephant, for example, is about four times as large as a man's, a blue whale's almost six times as large. Neither, of course, can match the forty-to-one body-to-brain-weight ratio in humans, but if ratios were all that mattered, the lowly field mouse, with a body-to-brain ratio of eight-to-one, would sit at the head of the class. Although the thinking part of the cerebrum, its outer shell, is four times thicker in humans than in rats, and four hundred times greater in surface area, the difference between men and mice is several orders of magnitude larger than dimensions alone can explain. It is not so much a matter of size, as of cerebral specialization. As the Alexandrian physician Erasistratus guessed in the fourth century b.c., the advantage lies in the folds, which are more developed in man than in any of the beasts.(2)
Are the folds in the brains of geniuses different from the folds of ordinary folk? The possibility has haunted investigators for a century and a half, and still has its supporters. Although not the only candidate for the anatomical substrate of genius, the folds are easily the front-runner because, contrary to the writings of the ancient anatomists, they are not entirely random. And where there is a pattern, there is assumed to be a meaning. In order to appreciate these patterns, you will not need a medical degree and a copy of Gray's Anatomy. To navigate the brain's tortuous surface, a thumbnail sketch should suffice.
To appreciate the rudimentary topology of the folds of the brain, place your right hand on the table and make a loose fist, relaxing the index finger so that the tip of the thumb rests inside the crux of the first knuckle. In other words, turn your hand into a talking clam, a puppet. Now shut the clam's mouth and imagine the hand enclosed in a mitten. What you are looking at, roughly speaking, is the left cerebral hemisphere of a primate brain. To turn it into a human brain, exchange for the mitten a boxing glove.
The surface of a real brain, of course, is riven by fissures, but these can easily be supplied with a pen. Begin by drawing a heavy line across the knuckles. In a real brain, this line is called the central sulcus (the Latin word for fissure); in older texts it is called the fissure of Rolando, after the eighteenth-century Italian anatomist who first described it. Just in front of this line is the precentral gyrus (gyri, also known as convolutions, are ridges of tissue that lie between fissures); just behind the precentral gyrus lies another ridge called the postcentral gyrus. The first of these contains the motor cortex, which in the left hemisphere controls movement on the right side of the body. The arrangement is inverted—the highest part of the gyrus, nearest the crown of the head, controls the foot, then comes the leg, and so on down to the lowest part of the gyrus, near the temple, which controls the hand and face. The postcentral gyrus registers sensation in a similar mapping. Taken together, these two convolutions, running over the crown of the head, form what is called the sensorimotor cortex.
Another important fissure, the lateral or Sylvian fissure, does not need to be drawn. It coincides with the gap between the thumb and the hand in the boxing glove model, and like that gap, it is very deep. A third useful line of reference, the occipital sulcus, should be added to the picture as a light line running across the very back of the hand, an inch or so above the wrist. Although there are a few other important fissures, these three—the central, Sylvian, and occipital—allow the hemisphere to be divided into its principal parts, the lobes.
The division of the hemispheres into lobes did not come about until the 1850s, and is generally credited to a French anatomist named Louis-Pierre Gratiolet.(3) It may come as a shock to discover that the lobes are not separate and independent units, that Gratiolet divided them somewhat arbitrarily and named them out of convenience by borrowing the words that describe the adjoining bones of the skull. There are four of them—the frontal, parietal, temporal, and occipital bones—delimited by the sutures, the skull's clearly visible expansion joints. Before Gratiolet came along, anatomists referred to the lobes of the brain using descriptors such as "posterior" and "anterior." But what Gratiolet noticed, after comparing hundreds of primate brains, was that although the pattern of folds in each brain is unique, there is a noticeable regularity to each one, especially in the deep fissures, much like the lines on the palm of the hand. The three primary fissures noted above, Gratiolet discovered, exist in the otherwise smooth brains of apes. They are also the first fissures to develop in the human fetus. It seemed only natural to use them as boundary lines for the four lobes. (A fifth lobe, tucked in the deep fold of the Sylvian fissure, completed the picture. Gratiolet called it the central lobe, although it is now known as the insula, and also, rather evocatively, as the island of Reil.)
The four principal lobes are easy to locate on the glove model of the brain. The frontal lobe lies above the Sylvian fissure and in front of the central sulcus. It is, essentially, the part of the boxing glove that encloses the fingers. Behind the central fissure lies the parietal lobe (that is, the back of the hand). The thumb of the glove, below the Sylvian fissure, represents the temporal lobe, and it extends as far back as the knuckle nearest the thumb's web. The base of the thumb, including its large pad of muscle, stands in for the occipital lobe. The dividing line is the occipital fissure. To picture the insula, imagine a handkerchief tucked into the fist, as in a magician's sleight of hand, and open up the thumb to reveal it.
Pierre Gratiolet did not believe that his division of the hemispheres into lobes had any functional significance. He preferred to think that the cortex worked more or less as a unit. But within ten years of his death in 1865, he would be proved wrong. During the 1870s, researchers in Germany and England produced detailed maps of cortical functions by stimulating the exposed brains of dogs, cats, and monkeys with electrical shocks. Through painstaking (if not pains-giving) labors they succeeded in correlating regions to reactions, and in the process set off a worldwide humane movement of antivivisectionists. At the same time, and less controversially, other researchers attempted to match clinical observations of neurological symptoms with postmortem examinations of brain lesions and tumors (a field known as morbid anatomy). In 1861, for example, Gratiolet's friend Paul Broca, a pioneering surgeon, demonstrated two instances of damage to the third frontal convolution in the left hemisphere in patients suffering from aphasia, a condition marked by the inability to produce speech. A decade later, the German psychiatrist Carl Wernicke isolated several other cases in which damage to the temporal lobe destroyed the ability to comprehend spoken or written language, while speech production remained unimpaired.
A quick consultation of the fist model of the brain shows a correspondence between the proximal phalanx of the index finger and the third frontal convolution of the cortex. This is Broca's area, a left-brain module (in right-handed people) responsible for the mechanical production of speech. Below it, in the web of the thumb (that is, in the temporal lobe), is Wernicke's area, which handles language processing and speech recognition.
Through similar investigations the visual cortex was found to be situated in the occipital lobe, and the acoustic areas in the temporal lobes, but the experiments went only so far. There remained (and still remain) areas in each lobe, especially in the frontal and parietal lobes, that are not assigned motor or sensory roles. These have been lumped together as the so-called association areas, and are believed to be connected with memory, spatial and temporal orientation, and the intellect.
Keep in mind that your actual hand, the right fist you are looking at, is controlled by a relatively large part of the motor cortex in the left hemisphere it represents, a fact noted even by some of the ancients. This contralateralism—left controlling right, and vice versa—arises in the spinal cord at a splitting and crossing just below the brain stem in a place known, rather exotically, as the decussation of the pyramids. This is where the brain ends. Or at least it is where the pathologist cuts the spinal cord when removing it.
Given the complexity of what lies beneath, the outward aspect of the cerebral hemispheres, especially in a preserved brain, is deceiving. Its bloodless, somewhat bloated, aspect suggests a solid, homogeneous core—a cauliflower, perhaps. The business of thinking, it might seem, goes on somewhere in the middle. But as it turns out, the real action takes place near the surface. What goes on inside is a good deal of networking.
If you take a wide blade and cut straight through a brain, the resulting cross section will reveal a thin beige rind bordering vast estuaries of pink. Once fixed in formaldehyde, the picture becomes clearer. The rind turns gray, giving rise to the popular term "gray matter," and the pink fades to white, becoming "white matter." The gray matter is the thinking apparatus. Only four millimeters thick in most places, it consists of nerve cells, or neurons, and support cells known as glial cells, in numbers so staggering that they can only be roughly estimated, like the grains of sand on a beach. In its entirety, the gray matter is usually referred to as the cerebral cortex or isocortex--some 30 billion (give or take a few hundred million) neurons worth of computational hardware that accounts for almost half of the neurons in the entire brain. (Surprisingly, the cerebellum, which has only a third of the surface area of the cerebrum, contains even more neurons in its own gray matter.)