A User's Guide to the Brain: Perception, Attention, and the Four Theaters of the Brain
By John J. Ratey, M.D.
As we have already learned, the brain is a dynamic ecosystem. The various neurons and networks are engaged in fierce competition for incoming stimuli. Networks that succeed in processing new experiences or behaviors end up as strong, permanent members of the neuronal neighborhood, while unused networks, cut off from the ebb and flow of information, wither away and die. In effect, the brain's structure becomes the information that it receives, and so how it perceives that information determines its future state. The adage we introduced in Chapter 1 applies equally well to perception: Use it or lose it. We must use the senses and their neurons or lose them forever to premature death or to be recruited for another function.
It is easy to understand how a disturbance in hearing, vision, or touch might impair normal brain development. The brain is constantly receiving information about its current state, both from the senses, concerning events in the environment, and from internal messages about the position of the body, its level of arousal, the activities of the various organs, and the chemical and nutritive state of the blood. Because the brain seeks to maintain a condition of internal constancy (homeostasis) in the face of a changing world, it is constantly interpreting all these incoming stimuli as instructions to modify the levels of neurotransmitters and hormones, die rates of electrical firing, and the chemical excitability of its own neural networks.
The development of personality itself is firmly rooted in the sensory apparatus. Even people with extraordinary perceptual abilities often exist in a love/hate relationship with their gifts, because it can be alienating to "see" the world differently than most people. Yet that different view is the defining characteristic of any great artist-indeed, the characteristic that makes each of us unique. The great American architect Buckminster Fuller, perhaps best known for his creation of the geodesic dome, often felt tremendously overloaded by visual stimulation. He would routinely wear glasses that allowed only part of the visual spectrum to enter his eyes, and when he put them on he found it easier to think. When out on a building site or out for a stroll in the city, he would wear earplugs because the noise left him unable to deal with the world.
Understanding how we see, hear, touch, smell, and even taste the world can tell us a lot about how we function in it. The sensations - or qualia - that come in from the environment are fitted into the categories or constructs that we have learned. We are constantly priming our perceptions, matching the world to what we expect to sense and thus making it what we perceive it to be. The first unsuspecting bite of a chili pepper causes our mouths to burn. The next time we are dared to taste a pepper we begin sweating, and we proclaim it to be just as hot as the first one-even if a tricky lab technician in a taste laboratory has presented us with a milder one. In this booming era of discovery, we have learned that the brain's neural networks respond in a pattern that is established by past experience: the more often a specific pattern is fired in response to a stimulus, the more firm the nerve assembly becomes. Hence the axiom: Neurons that fire together wire together. Input shapes the way we experience the next input. It is not an exaggeration to state that after you have an experience, you are not the same person you were before the experience. Experience colors perception.
What happens to our brains during new experiences? First, we must reject the idea that our brains are static storage depots of information. Rather, the nerves are constantly making new connections that will serve us better in the things we do frequently. The brain can be shaped by experiences, just as particular muscles respond to particular exercises. As we rehearse lines in a play or memorize multiplication tables, we build nerve assemblies, just as we do when we repeat a dance step or karate move. As our brains train, the tasks become easier and more automatic.
This training is accomplished in a fascinating and intricate way. The brain's nerve cells self-organize when they have been trained enough by repeated contact with a particular stimulus. A baby learns to distinguish its mother's voice from those of others; a second violinist learns to pick out his part when listening to a symphony. Test subjects learn to see the world upside down for a day. The neurons become "primed," prejudiced to expect the same old song. When they meet unfamiliar stimuli, they perceive the input as new and disturbing. Thankfully. For the disturbance leads to reorganization, which is why Rickie was able to recover and why we can delight in a new pattern of sunset on a new day.
Evidence of the brain's ability to adjust perception was one of the early influences that led me to a career in neuropsychiatry. In 1958 my brother applied for admission into the Naval Academy but was sent back because his vision was not perfect. However, they said he could reapply if his vision improved, and they gave him eye exercises to develop more visual acuity. A month later he passed the entrance eye exam. The exercises had trained his eye muscles and his brain's visual circuits. This was in 1959, long before most of us knew much about the plasticity of the brain.
An act of perception is a lot more than capturing an incoming stimulus. It requires a form of expectation, of knowing what is about to confront us and preparing for it. Without expectations, or constructs through which we perceive our world, our surroundings would be what William James called a "booming, buzzing confusion" and each experience truly would be a new one, rapidly overwhelming us. We automatically and unconsciously fit our sensations into categories that we have learned, often distorting them in the process.
For example, our "coherent" vision of the world actually comes from millions of bits of fragmented visual information. Even as you stare at this sentence, your eyes are constantly darting a bit in various directions, rarely focusing on any word or letter for more than a split second. Furthermore, the rest of your peripheral vision is fuzzy. That's because only one tiny pinhole region in the center of the eye, the fovea, can see with absolute clarity. In the fovea, the photoreceptors known as cones are jammed together, thus making their message loud and clear. The photoreceptors in the periphery, on the other hand, are more dispersed, so the messages they send are less clear. If you concentrate on your peripheral vision, you'll notice that there is hardly any detail. Yet we perceive our visual environment as a seamless, detailed reality all around us. How? The retina splits incoming information into specialized systems that carry only specific types of details, like a special highway up to the brain for motion, color, form, and so forth. Why? Because our brains would overload with visual clutter without the retina acting as a triage of sorts.
The fovea sees with absolute clarity only a thumbnail-size portion of a scene. It sees only bits of shapes, portions of curves, sections of edges, and parts of colors. It does not see whole shapes or colors. The brain predicts final shapes from the fragmented parts that the fovea sees. Nerve impulses that reflect fragments of images, movements, and wavelengths are sent to visual memory centers in the brain, which contain permanently stored image patterns. If the fovea's fragmented image can be matched to a pattern stored in the memory center, voilà!-the object is recognized.
The brain's need to predict, in order to fill the gaps between the fragments of images we see, is also the very reason we are prone to visual illusions. We think we see something that isn't there because the cues trigger our prediction models to tell us that it is. This is what magicians and sleight-of-hand experts depend on. We fill in visual information all the time. Each of us has a blind spot in our visual field that occurs because the place where the optic nerve comes into the retina has no rods or cones. We do not see in a big area of our field of vision in either eye. Having two eyes, and thus binocular vision, makes up for this. However, if you cover one eye, you do not see the scene in front of you with a hole in the middle, because our brain fills in this blind spot and does it well. We also fill in details and patterns all the time; when we spot our dog through a lattice fence, we do not see just parts of the dog but experience its whole visual image.