Your Brain—How Does It Work?
“The brain is the most difficult part of the body to study,” observes E. Fuller Torrey, a psychiatrist at the U.S. National Institute of Mental Health. “We carry it around in this box on our shoulders that’s very inconvenient for research.”
NEVERTHELESS, scientists say that they have already learned much about the way the brain processes the information that our five senses supply. Consider, for example, the way it deals with visual sensations.
Your Mind’s Eyes
Light reaches your eye and strikes the retina, consisting of three layers of cells at the back of your eyeball. Light penetrates to the third layer. This layer contains cells known as rods, which are sensitive to brightness, and cones, which are responsive to light of different wavelengths corresponding to the colors red, green, and blue. The light bleaches pigment in these cells. This sends a signal to cells in the second layer and from there to other cells in the top layer. Axons of these cells combine to form the optic nerve.
The millions of neurons of the optic nerve arrive at a junction in the brain known as the optic chiasma. Here neurons carrying signals from the left-hand part of each eye’s retina now meet and follow parallel tracks to the left-hand side of the brain. Similarly, signals from the right-hand side of each retina join forces and travel to the right-hand side. The impulses arrive next at a relay station in the thalamus, and from there the next neurons pass the signals to the area at the back of the brain known as the visual cortex.
Different aspects of visual information travel along parallel paths. Researchers now know that the primary visual cortex together with a nearby region acts like a post office in sorting, routing, and integrating the variety of information that the neurons bring. A third region detects shape, such as the edge of an object, and motion. A fourth area recognizes both form and color, whereas a fifth one constantly updates maps of the visual data to track movement. Current research indicates that as many as 30 different brain areas process the visual information the eye collects! But how do they combine to present you with an image? Yes, how does your mind “see”?
“Seeing” With One’s Brain
The eye gathers information for the brain, but it is the cortex that evidently processes the information that the brain receives. Take a picture with a camera, and the resulting photo reveals details of the whole scene. But when your eyes observe the same view, you consciously observe only that part of the scene on which you focus your attention. How the brain does this remains a mystery. Some believe that it is the result of a stage-by-stage integration of visual information in so-called convergence zones, which help you compare what you see with what you already know. Others suggest that when you fail to see something in plain view, it is simply because the neurons controlling attentive vision are not firing.
Whatever the case, the difficulties scientists have in explaining vision pale in comparison with the problems faced in determining just what “consciousness” and the “mind” really involve. Scanning techniques, such as magnetic resonance imaging and positron-emission tomography, have provided scientists with a new window on the human brain. And by observing the flow of blood to certain brain areas during thought processes, they have concluded with reasonable certainty that different regions of the cortex apparently help one to hear words, see words, and speak words. However, as one writer concludes, “the phenomenon of mind, of consciousness, is much more complex . . . than anyone suspected.” Yes, much of the brain’s mystery has yet to be unraveled.
The Brain—Just a Marvelous Computer?
To understand our complex brain, it may be helpful to make comparisons. At the beginning of the industrial revolution, in the mid-18th century, it became fashionable to compare the brain to a machine. Later, when telephone switchboards became a mark of progress, people compared the brain to a busy switchboard with an operator who made decisions. Now that computers handle complicated tasks, some compare the brain to a computer. Does this comparison fully explain how the brain works?
Significant basic differences separate the brain from a computer. Fundamentally, the brain is a chemical system, not an electrical one. Numerous chemical reactions occur within each cell, and this is totally different from the workings of a computer. Also, as Dr. Susan Greenfield observes, “no one programmes the brain at all: it is a proactive organ, operating spontaneously.” This is unlike a computer, which has to be programmed.
Neurons communicate with one another in a complicated way. Many neurons react to 1,000 or more synaptic inputs. To grasp what this involves, consider the research of one neurobiologist. He studied an area on the brain’s underside just above and behind the nose to discover how we recognize odors. He notes: “Even this apparently simple task—which seems a pushover compared to proving a geometric theorem or understanding a Beethoven string quartet—involves about 6 million neurons, each one receiving perhaps 10 000 inputs from its mates.”
The brain is, however, more than a collection of neurons. For every neuron, there are several glial cells. In addition to holding the brain together, they provide electrical insulation for the neurons, fight off infection, and join together to form a protective blood-brain barrier. Researchers believe that the glial cells may have other functions that are yet to be discovered. “The obvious analogy to man-made computers, which process electronic information in digital form, may be so incomplete as to be misleading,” concludes the Economist magazine.
This still leaves us with another mystery to discuss.
What Are Memories Made Of?
Memory—“perhaps the most extraordinary phenomenon in the natural world,” according to Professor Richard F. Thompson—involves several different functions of the brain. Most students of the brain divide memory into two kinds, declarative and procedural. The procedural involves skills and habits. The declarative, on the other hand, involves storing facts. The Brain—A Neuroscience Primer itemizes memory processes according to the time they take: very short-term memory, which lasts about 100 milliseconds; short-term memory, which is of a few seconds’ duration; working memory, which stores recent experiences; and long-term memory, which houses verbal material that has been rehearsed and motor skills that have been practiced.
One possible explanation of long-term memory is that it starts with activity in the front part of the brain. The information chosen for long-term memory passes as an electrical impulse to a part of the brain known as the hippocampus. Here a process called long-term potentiation enhances the neurons’ ability to pass messages.—See the box “Bridging the Gap.”
A different theory of memory stems from the idea that brain waves play a key part. Its proponents believe that regular oscillations of the brain’s electrical activity, rather like the beat of a drum, help bind memories together and control the moment at which different brain cells are activated.
Researchers believe that the brain stores different aspects of memories in different places, each concept being linked to the area of the brain that specializes in perceiving it. Some parts of the brain certainly contribute to memory. The amygdala, a small almond-size clump of nerve cells close to the brain stem, processes memories of fear. The basal ganglia region is focused on habits and physical skills, and the cerebellum, at the base of the brain, concentrates on conditioned learning and reflexes. Here, it is believed, we store the skills of balance—for example, those we need to ride a bicycle.
Our brief glimpse of how the brain works has necessarily omitted details of other remarkable functions, such as its timekeeping, its propensity for acquiring language, its intricate motor skills, and its way of regulating the body’s nervous system and vital organs and of coping with pain. Then, still being discovered are its chemical messengers that link with the immune system. “The complexity is so incredible,” observes neuroscientist David Felten, “that you wonder if there is ever any hope of working it out.”
Although many of the brain’s mysteries remain unsolved, this remarkable organ provides us with the capacity to think, to meditate, and to recall what we have already learned. But how can we make the best use of the brain? Our concluding article in this series provides an answer.
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BRIDGING THE GAP
When a neuron is stimulated, a nerve impulse travels along the axon of the neuron. On reaching the synaptic bulb, it causes tiny globules (synaptic vesicles), each holding thousands of neurotransmitter molecules, which are within the bulb, to fuse with the bulb’s surface and release their cargo across the synapse.
Through a complicated system of keys and locks, the neurotransmitter opens and closes input channels in the next neuron. As a result, electrically charged particles flow into the target neuron and cause additional chemical changes that either spark an electrical impulse there or inhibit further electrical activity.
A phenomenon called long-term potentiation occurs when neurons are regularly stimulated and release neurotransmitters across the synapse. Some researchers believe that this draws the neurons closer together. Others claim that there is evidence that a message feeds back from the receiving neuron to the transmitting neuron. This, in turn, causes chemical changes that produce yet more proteins to serve as neurotransmitters. These then strengthen the bond between the neurons.
The changing connections in the brain, its plasticity, give rise to the motto, “Use it or lose it.” Thus, to retain a memory, it is helpful to recall it often.
A signal-carrying fiber that links neurons
Short, many-branched connections that link neurons
Tentaclelike projections from the neuron. There are two main types—axons and dendrites
Nerve cells. The brain has about 10 billion to 100 billion neurons, “each connected to hundreds, sometimes thousands, of other cells”
Chemicals that take a nerve signal across the so-called synaptic gap between a sending nerve cell, or neuron, and a receiving one
The gap between a sending and a receiving neuron or nerve
Based on The Human Mind Explained, by Professor Susan A. Greenfield, 1996
CNRI/Science Photo Library/PR
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HUMANS’ DISTINCTIVE ABILITIES
Specialized areas of the brain known as language centers equip humans with remarkable skills of communication. What we want to say appears to be organized by the region of the left brain hemisphere known as Wernicke’s area (1). This communicates with Broca’s area (2), which applies grammatical rules. Impulses next arrive at nearby motor areas that control facial muscles and help us form appropriate words. Additionally, these areas connect with the brain’s visual system so that we can read; with the hearing system so that we can hear, understand, and respond to what others tell us; and, not to be neglected, with our memory bank to store worthwhile thoughts. “What really sets humans apart from other animals,” comments the study guide Journey to the Centres of the Brain, “is their ability to learn an astonishing variety of skills, facts and rules, not just about physical things in the world around them, but especially about other people and what makes them tick.”
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Different areas of the brain process color, form, edge, and shape and also track movement
Parks Canada/J. N. Flynn