Evolution has created a staggering range of organisms, each with features cleverly honed for its environmental niche. But while evolution is a fantastic creator, adding almost whatever is needed, it is surprisingly lazy at tidying up after itself, at pruning what is no longer required. In the bacterial world, where margins for survival may be razor sharp, things are more efficient. But most animals carry with them a surplus of obsolete features, such as the astronomical quantities of pathological DNA interlopers that sit in every cell in our bodies. But there are also more large-scale examples of detritus we endure. For instance, whenever we get cold, our hair duly stands on end to create a buffer of trapped air around our skins, as if such an action would make any difference to keep us cocooned from the cold—it doesn’t (unlike other primates, we simply don’t have enough hair to make this automatic response functional).
The human brain is in some ways an even more extreme example of a process that is far more creative than destructive. We effectively have three evolutionary versions of brains in our heads. Our brains are rather like a city that has existed since ancient times. In Cambridge, for instance, the historic center is squashed inside a fertile bend in the river Cam. This is the core of the city. Here there used to be a castle on a small hill, originally built by William the Conqueror in the eleventh century. The oldest parts of the university, along with old churches and so on, are still there. Over the centuries, housing, university colleges, and research departments have sprung up around this central district. And now, around this second band of somewhat old structures, there are the outer suburbs, with modern housing along with large technology and business parks. Although an unromantic person might be tempted to replace the oldest buildings of the city and the narrow winding roads of the core area with efficient modern streets and buildings, all these ancient places still serve some purpose today. The expense of such renovations simply wouldn’t be worth the trouble.
At the center of our brains, too, lies the oldest part, mainly involving the brain stem. This is sometimes called the “reptilian brain” because it is the only region we share with our reptilian ancestors. The brain stem is the gateway between the brain and the body—all sensory signals from the body, from the stroke of a lover on our faces to the pinprick of a needle in our arms, passes through the brain stem to the rest of the brain. In turn, all commands from more sophisticated parts of the brain—for instance, for us to tango or kick a football—are shunted through the brain stem, down into the spinal cord, and then through to the rest of our bodies to make our motor commands seamlessly fulfilled. The brain stem also controls other basic functions, such as our breathing and heart rate.
Because the brain stem is such a critical part of our brains, any damage to it due to stroke, tumor, or some accident is usually extremely serious. Damage can frequently lead to death, or, if not death, then a permanent loss of consciousness. Since all motor commands pass through this tiny part of the brain, damage can also lead to a rare but chilling condition known as “locked-in syndrome,” where a patient might be normally conscious, but almost entirely paralyzed.
One sufferer, Jean-Dominique Bauby, produced a very vivid description of this state. At the time of his stroke in 1995 he was the editor of Elle magazine in France. His stroke left him in a coma, from which he completely recovered twenty days later, at least mentally. He found upon waking that he was utterly unable to move any part of his body except his eyes—and his head, a little. Staggeringly, by just the blinking of his left eyelid and the calm assistance of a transcriber using an alphabet board to ascertain what letter he wanted to spell next, he wrote a book, The Diving Bell and the Butterfly, an impressive memoir about his former life and his subsequent mental prison.
Surrounding the brain stem is the limbic system, which is our evolutionary link to the earliest mammals. This system could almost be called our instinct center. It is here where our sexual orientation and proclivities are determined. Also, our hunger and thirst are partly controlled here. Our temperature is regulated in the limbic system, and our biological clocks tick away via rhythmic neurons in these limbic nuclei. Our primitive emotions, such as rage and fear, are generated here, as are our instinctive reactions to fight or to flee. One interesting hint of the age of this region can be found in the kind of phobias we as humans tend to have. Despite the thousands of deaths every year from car accidents, how many people do you know who are terrified of a Ford pickup? And yet, spider bites almost never cause fatalities anymore, but you almost certainly know someone (maybe even yourself) who just can’t bear creepy crawlies. The simple reason for this is that millions of years ago, spiders really were serious threats to life. Again, evolution can sometimes be very lazy about updating.
One special region to mention within this class of intermediate brain structures is the thalamus. This is effectively a relay station, the Grand Central Station of brain regions. It connects virtually all parts of the brain together. With damage to certain parts of it, but with everything else intact, a person will enter a vegetative state and show very little sign of consciousness. The thalamus is an important structure to support our experiences, possibly by allowing information to flow smoothly across every corner of the brain.
One reason that our instinctive fears have not been updated is that many aspects of modern life, such as cars, are just too new for natural selection to have yet found any traction. But another reason concerns our third, most modern brain region, the cortex, which is the outer shell surrounding the rest of the brain (cortex comes from the Latin for “bark”). Only modern mammals have this new neural toy. Evolution doesn’t need to update our innate fears because we have such a powerful information-processing mechanism in the cortex. We can learn any new fear we need, and even suppress existing ones, if necessary. The cortex is where our most complex, flexible mental activity resides. It duplicates and can potentially modify and control many of the functions of the other two more primitive portions of our brain, but the cortical version of such functions is invariably far more sophisticated.
The cortex, too, exhibits a surprising degree of redundancy. It comprises four main sections, or “lobes”, but each lobe has a twin on the other half (or hemisphere) of the brain. The occipital lobes, rather bizarrely located at the back, furthest from the eyes, largely process vision. The temporal lobes, at the bottom middle of the brain, process hearing and some aspects of language (especially in the left hemisphere), but are also where simple visual processing becomes object recognition. This is where most of our long-term memories live—storage of the faces we’ve seen in our lives, our history and sense of meaning. The parietal lobes, at the top back, help process our sense of space, as well as of touch. As part of their spatial representation, the parietal lobes may help us represent numbers, and contribute to short-term memory, via holding on to more than one item in space. They have been linked to the ability to boost attention to some detail in the world. But, increasingly of late, the back portion of the parietal lobes have more generally been linked with complex thought, such as when we attempt an IQ test.
In some ways, the frontal lobes are the odd ones out. They aren’t devoted to any sensory processing (although they are next to our smell center, which is found at the front lower section of the brain). You can remove a large section of the frontal lobes in one hemisphere and not see any obvious impairment in your patient (the exception to this is right at the back of the frontal lobes, where the main motor strips are located, but we’ll ignore this part for the moment). After losing a substantial proportion of his right frontal lobe, for instance, a patient will in all likelihood still be able to move normally, will have intact senses, and will suffer little, if any, memory loss. He lost the frontmost section of his frontal lobes, and his personality was radically altered as a result. This region, known as the orbitofrontal cortex, is now thought to be a secondary emotion center, supplementing the more primitive emotions of the limbic system, the second-oldest evolutionary band of human neural territory. It is in the orbitofrontal cortex that our most complex emotions are activated, such as how we feel and act in social settings and how we convert levels of risk or reward into decisions to act.
Much of the rest of the frontal lobes is responsible for our most abstract thoughts. This area has most closely been associated with IQ, as well as with virtually any task that is either very complex or novel.
The frontal lobe, especially the mid outer part, known as the lateral pre-frontal cortex, is thought most centrally to be involved in conscious processes, probably in concert with the posterior parietal cortex, which shares many of its functions.
Before moving on, I should clarify a couple of details. First, by talking about a reptilian brain I’m not suggesting that all the animals from which we diverged earlier in our common histories are so primitive as to be unable to learn in potentially sophisticated ways. Any animal that we became estranged from hundreds of millions of years ago has had ample evolutionary time to develop advanced neural weapons on its own.
Second, these “three brains” living within us aren’t in any way independent. They normally work together, in a highly interconnected way, for the common purpose of keeping us alive and thriving. Each level can take command if necessary, and sometimes there is competition between stages within this hierarchy. For instance, we might find ourselves sprinting for our lives, because our fear center, the amygdala in the limbic system, has swiftly taken command after we spotted a poisonous snake in the jungle. For us to sprint away, this intermediate limbic system is using the modern cortical instructions to hurry away, along with the brain-stem instructions to breathe faster, to inject us with lots of energy in order to help us speed away from the danger. But we could also, later on, employ our modern cortex to return cautiously to the snake. We might suppress the intermediate limbic fear messages and deliberately calm our breathing via cortical messages to our brain stem, all so that we might examine this interesting serpent from a safe distance.
As most of the basic functions of our reptilian brains carry on unabated when we lose consciousness, we can safely say that whatever awareness is for, it isn’t concerned with those basic internal processes, like breathing, that are vital to our survival. Likewise, consciousness probably has little to do with the brain stem. However, our two remaining brain layers, our limbic brain and our cortices, enter a special noncommunicative state during anesthesia, making consciousness impossible.
Excerpted with permission from “The Ravenous Brain: How the New Science of Consciousness Explains Our Insatiable Search for Meaning,” by Daniel Bor. Available from Basic Books, a member of The Perseus Books Group. Copyright © 2012.
No comments:
Post a Comment