Evolution has provided us with an unconscious mind because our unconscious is what allows us to survive in a world requiring such massive information intake and processing. Our sensory perception, our memory recall, our everyday decisions, judgments, and activities all seem effortless—but that is only because the effort they demand is expended mainly in parts of the brain that function outside awareness.
Take speech. Most people who read the sentence “The cooking teacher said the children made good snacks” instantly understand a certain meaning for the word “made.” But if you read, “The cannibal said the children made good snacks,” you automatically interpret the word “made” in a more alarming sense. Though we think that making these distinctions is easy, the difficulty in making sense of even simple speech is well appreciated by computer scientists who struggle to create machines that can respond to natural language. Their frustration is illustrated by a possibly apocryphal story of the early computer that was given the task of translating the homily “The spirit is willing but the flesh is weak” into Russian and then back into English. According to the story, it came out: “The vodka is strong but the meat is rotten.” Luckily, our unconscious does a far better job, and handles language, sense perception, and a teeming multitude of other tasks with great speed and accuracy, leaving our deliberative conscious mind time to focus on more important things, like complaining to the person who programmed the translation software. Some scientists estimate that we are conscious of only about 5 percent of our cognitive function. The other 95 percent goes on beyond our awareness and exerts a huge influence on our lives—beginning with making our lives possible.
One sign that there is a lot of activity going on in our brains of which we are not aware comes from a simple analysis of energy consumption.10 Imagine yourself sprawled on the couch watching television; you are subject to few demands on your body. Then imagine yourself doing something physically demanding—say, racing down a street. When you run fast, the energy consumption in your muscles is multiplied by a factor of one hundred compared to the energy you use as a couch potato. That’s because, despite what you might tell your significant other, your body is working a lot harder—one hundred times so—when you’re running than when you’re stretched out on the sofa. Let’s contrast this energy multiplier with the multiplier that is applicable when you compare two forms of mental activity: vegging out, in which your conscious mind is basically idle, and playing chess. Assuming that you are a good player with an excellent knowledge of all the possible moves and strategies and are concentrating deeply, does all that conscious thought tax your conscious mind to the same degree that running taxed your muscles? No. Not remotely. Deep concentration causes the energy consumption in your brain to go up by only about 1 percent. No matter what you are doing with your conscious mind, it is your unconscious that dominates your mental activity—and therefore uses up most of the energy consumed by the brain. Regardless of whether your conscious mind is idle or engaged, your unconscious mind is hard at work doing the mental equivalent of push-ups, squats, and wind sprints.
ONE OF THE most important functions of your unconscious is the processing of data delivered by your eyes. That’s because, whether hunting or gathering, an animal that sees better eats better and avoids danger more effectively, and hence lives longer. As a result, evolution has arranged it so that about a third of your brain is devoted to processing vision: to interpreting color, detecting edges and motion, perceiving depth and distance, deciding the identity of objects, recognizing faces, and many other tasks. Think of it—a third of your brain is busy doing all those things, yet you have little knowledge of or access to the processing. All that hard work proceeds outside your awareness, and then the result is offered to your conscious mind in a neat report, with the data digested and interpreted. As a result, you never have to bother figuring out what it means if these rods or those cones in your retinas absorb this or that number of photons, or to translate optic nerve data into a spatial distribution of light intensities and frequencies, and then into shapes, spatial positions, and meaning. Instead, while your unconscious mind is working feverishly to do all those things, you can relax in bed, recognizing, seemingly without effort, the lighting fixture on the ceiling—or the words in this book. Our visual system is not only one of the most important systems within our brain, it is also among the most studied areas in neuroscience. Understanding its workings can shed a lot of light on the way the two tiers of the human mind function together—and apart.
One of the most fascinating of the studies that neuroscientists have done on the visual system involved a fifty-two-year-old African man referred to in the literature as TN. A tall, strong-looking man, a doctor who, as fate would have it, was destined to become renowned as a patient, TN took the first step on his path to pain and fame one day in 2004 when, while living in Switzerland, he had a stroke that knocked out the left side of a part of his brain called the visual cortex.
The main part of the human brain is divided into two cerebral hemispheres, which are almost mirror images of each other. Each hemisphere is divided into four lobes, a division originally motivated by the bones of the skull that overlie them. The lobes, in turn, are covered by a convoluted outer layer about the thickness of a formal dinner napkin. In humans, this outer covering, the neocortex, forms the largest part of the brain. It consists of six thinner layers, five of which contain nerve cells, and the projections that connect the layers to one another. There are also input and output connections from the neocortex to other parts of the brain and nervous system. Though thin, the neocortex is folded in a manner that allows almost three square feet of neural tissue—about the size of a large pizza—to be packed into your skull.11 Different parts of the neocortex perform different functions. The occipital lobe is located at the very back of your head, and its cortex—the visual cortex—contains the main visual processing center of the brain.
A lot of what we know about the function of the occipital lobe comes from creatures in which that lobe has been damaged. You might look askance at someone who seeks to understand the function of the brakes on a car by driving one that doesn’t have any—but scientists selectively destroy parts of animals’ brains on the theory that one can learn what those parts do by studying animals in which they no longer do it. Since university ethics committees would frown on killing off parts of the brain in human subjects, researchers also comb hospitals seeking unfortunate people whom nature or an accident has rendered suitable for their study. This can be a tedious search because Mother Nature doesn’t care about the scientific usefulness of the injuries she inflicts. TN’s stroke was noteworthy in that it pretty cleanly took out just the visual center of his brain. The only drawback—from the research point of view—was that it affected only the left side, meaning that TN could still see in half his field of vision. Unfortunately for TN, that situation lasted for just thirty-six days. Then a tragic second hemorrhage occurred, freakishly destroying what was almost the mirror image of the first region.