William H. Calvin and George A. Ojemann, Inside the Brain: Mapping the Cortex, Exploring the Neuron (New American Library, 1980), chapter 1. See also http://WilliamCalvin.com/Bk1/bk1ch1.htm.
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William H. Calvin
University of Washington
|Different views of the human brain, by Anthony Ravielli (reproduced with permission from The Human Brain, Its Capacities and Functions, by Isaac Asimov [New York. Mentor Books, 1963]).|
William H. Calvin and
George A. Ojemann
The neurosurgery operating room is a little unusual even from the outside. On its swinging doors today hangs a prominent sign proclaiming, "Quiet Please, Patient Awake." Most surgical patients get to sleep while the OR crew works, hence the reminder to save the OR jokes for another day.
The OR door is a heavy, solid door that does not push open easily except by the surgeon's trick of backing into it. Surgeons have to do that anyway, to keep from touching it with their dripping hands, fresh from the scrub sink outside. Nondripping types sometimes just heave with their shoulders.
Inside, there is all the usual paraphernalia. Tables covered with unironed green tablecloths, the sterile drapes on which the usual array of instruments have been laid out by the scrub nurse. There is the anesthesiologist's cart with air tanks and respirator devices, even though general anesthesia is not being used today. And today there is even more than the usual quota of TV and electronics gear.
The patient is lying on his right side, a doughnut-shaped pillow under his head so that his right ear won't get sore. Neil will be lying in this position all day, although there are always breaks in the operation when he will be able to move around a little on the table and get comfortable again. The operating table is a well-padded couch with infinite variations in possible positions.
The surgeon is looking down on the left side of Neil's head. Earlier in the morning, a U-shaped incision was made in the skin and the scalp folded back over Neil's left ear. That's all the local anesthetic was needed for, since the deeper tissues are not particularly sensitive to pain. Getting through the bone is noisy but not painful. A piece of skull about the size of the patient's hand was removed, using air-driven drills and saws. The scrub nurse has the piece of skull lying on the instrument table, carefully wrapped up in moist sterile gauze, as it will be replaced in Neil's head at the end of the operation, along about dinnertime.
By this time in the middle of the morning, the neurosurgeon has also made a U-shaped incision in the dura, a tough membrane which covers the brain like a thin skin. Like the scalp flap, it too has been folded back and kept moist. For the first time, the brain itself has been exposed to the light of day, or, more precisely, to the big OR lights.
"Neil, now's a good time to move around a little. Is that comfortable for you? How are you feeling?" asks the neurosurgeon.
"That's a bit more comfortable. Oh, I'm feeling okay. How far along are you?" Neil answers.
"We're through with all the big mechanical parts of the surgery, Neil. Now we'll begin checking out the brain. Remember to let me know if you feel anything."
The neurosurgeon presses gently on the soft brain in various places, exploring to see if the brain tissue feels unusually tough, which might indicate a tumor, scar, or other pathological change in the brain tissue. Neil does not feel anything, which is perfectly natural, as the brain itself is insensitive to pain or touch; it is not equipped with the skin's type of transducer nerve cells, which specialize in sensing touch.
Fifteen years ago, Neil suffered a skull fracture in a car accident. He now has a type of epilepsy which the usual drugs have been incapable of controlling. The reason for the operation is that a long workup has indicated that the epileptic seizures seem to start in a particular area of the brain, the left temporal lobe, located just in front of Neil's left ear.
Neil is quite interested in all of this, and indeed is having an experience which few people will ever have. Neil is about to learn the precise areas in his own brain which control the movement of his hands and face, that receive the sensations from the transducer neurons in his skin; those areas of his brain which he uses to lift a fork, dial a telephone, play the piano, or speak to the neurosurgeon.
Neil is an engineer, a graduate of MIT. More than most patients who go through this operation, he can conceive of the human brain as something akin to a very elaborate computer, operating on electrical principles, with different regions of the brain specializing in different functions. Although Neil's brain has the same general plan as the brain of any other human being, the details of his brain differ from all others, just as his face is unlikely to be identical to anyone else's. The neurosurgeon needs to know what's where in Neil's brain, and he can't tell by merely looking at the surface of the brain.
"Neil, we are now getting to the Part of the operation I told you about earlier, where I am going to stimulate the brain electrically. You're probably going to feel something. Tell me where you feel it."
The neurosurgeon picks up a penlike device with two silver wires on its tip. Connected to an electrical stimulator, the device is controlled by an electrical engineer looking down from behind the windows of the gallery above the OR. When the two silver wires are touched gently to the surface of Neil's brain, several milliamperes of current flow through the brain, and then the wires are lifted off.
"Hey, I felt something on my face," Neil says with some surprise.
"Did your face move first?"
"No, it just felt funny-- like tingling.
"Well, let me try another spot." The neurosurgeon momentarily touches the wires to an adjacent part of the brain, slightly in front of the first spot.
"My mouth moved!"
The anesthesiologist, who has been watching Neil's face carefully, says, "It was just the right corner of his mouth. It pulled up for a second, and then relaxed."
The neurosurgeon places a little piece of sterile paper, with a number printed on it, on the brain at the site where the mouth movement was evoked to mark the spot temporarily. The electrical stimulation is then applied to another site, farther away from the ear and closer to the top of the head.
"Hey, my hand moved! Just like someone else moved it for me!"
Which is, of course, exactly what has happened. The part of Neil's left brain touched by the stimulating wires is part of the brain essential for movement of his right hand. Neil uses it whenever he wants to move his right hand. The electricity applied by the neurosurgeon simply bypasses the voluntary control and directly initiates the hand movement.
That the left side of the brain controls the right side of the body is something that was known even in the ancient civilizations. The great Greek physician Hippocrates observed that after injuries to one side of the head, it was often the opposite side of the body which became paralyzed or might be involved in a seizure. That there is a very orderly map in the left brain for movements of the right side was discovered only in the nineteenth century. Its details were described by a British neurologist, Hughlings Jackson. As the neurosurgeon moves the stimulating wires toward the top of the head, the sequence of face to hand will progress to chest and hips. More detailed "mapping" using this technique will reveal thumb and forefinger areas within the hand area. More space in the brain seems to be devoted to the fingers than to the leg. 2
Parallel to this "motor strip" controlling muscles is a strip where sensations are evoked. This "sensory strip" is just to the rear of the motor strip. Marching up it with the stimulating wires would have caused Neil's tingling sensation to move with the same general progression from face to hand to the rest of the body.
The motor and sensory strips have an average arrangement over many patients, which allows maps to be made for textbooks. But there is considerable variation between people in the details of this organization, much more than is apparent just from visual inspection of the brain's surface infoldings. A deep infolded groove in the brain, the central sulcus, separates the motor and sensory strips in the usual textbook pictures of an "average" brain. Neil doesn't have that anatomical dividing line. After Neil's electrical-stimulation mapping has been completed, the numbered tags indicating the motor strip lie on the surface of the brain right next to sensory points; no infolded region separates them. It is not presently known whether such detailed differences are important, perhaps reflecting the differences between the clumsy and the highly coordinated. But the differences in brain surface anatomy are certainly there, and that is one reason why neurosurgeons try to map out the brain physiologically in an awake patient, who can help by reporting what happens.
Below the motor and sensory areas, across a groove (the sylvian fissure) that marks the top of the temporal lobe, is a small region where the electrical stimulation will cause Neil to report buzzing noises: the auditory receiving area. If the left visual cortex located in the back of Neil's brain were to be simulated, he would report flashes of light. Those light flashes would appear in the right side of Neil's vision, for like motor function, vision is wired up in a crossed fashion so that everything we see to the right of our center of vision goes to the left brain. Since the surgery today does not require the back of Neil's brain to be exposed, this cannot be done in Neil but it is well known from studying other patients.
There are many methods which researchers use in animals to map out the connections from sense organs to the brain, and from the brain to the muscles. They are much more precise than the methods suitable for use on humans in the OR. As we have seen, electrical stimulation of the brain's surface produces rather general effects: the tingling of the face is often diffuse, the involuntary movement of the hand is often uncoordinated, the sounds evoked are noises rather than tones, the light flashes have no form. They are very helpful for the neurosurgeon in evaluating what's where in this particular person, but such crude electrical stimulation activates millions of nerve cells indiscriminately. Laboratory equipment allows one to see a limited, selective, orderly sequence of cell activity in monkey brains when the monkey is reaching for a banana.
One of the reasons that applied electricity can so effectively stimulate the brain is that the brain runs internally on electricity, much as does a modern digital computer. Rather than burning fossil fuels to run generators, humans generate electricity by transporting glucose (one form of sugar) in the blood to the brain, where the glucose runs an elaborate metabolic pathway. One effect is to charge batteries in each nerve cell. These batteries then provide the energy with which electrical computations are performed. So-called "brain waves" are an indirect indication of the brain's electrical cycling processes and they play a major role in analyzing Neil's epilepsy.
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