Hello again from the brain-news-hound;
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Pulsing electromagnets offer new method of mapping brain
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Copyright =A9 1996 Nando.net Copyright =A9 1996 N.Y. Times News Service
(May 21, 1996 00:17 a.m. EDT) A psychiatrist presses a small,=20
hand-held device over a patient's skull. There is a clicking sound and=20
the patient feels what has been described as a "clawlike" sensation or=20
skin being "drawn up." Usually it is not uncomfortable.
But bizarre experiences may follow. If the device -- a powerful,
fluctuating magnet -- is placed on a spot over a person's left ear,
experimenters say, his right thumb will begin to waggle. Move the=20
magnet back an inch and he gets a vivid sensation of the thumb moving,=20
but his eyes tell him that it remains still. When the magnet is placed=20
at the back of his head, say on the left side, he will perceive a huge=20
blind spot in his right eye.
Researchers say the device also produces mood changes -- several hours=20
of sadness, or happiness, depending on whether it is placed over the=20
right or left eyebrow. In other experiments, researchers say the=20
magnet has temporarily relieved depression, reduced the jagged=20
movements in people who have Parkinson's disease and eradicated=20
phantom limb pain in amputees.
Although it may sound like something advertised on late night=20
television, the technique, called transcranial magnetic stimulation,=20
or TMS, is being taken seriously by a small group of neurologists and=20
psychiatrists around the world who are using it as a research tool.
All the results are preliminary, and so far the technique has been=20
used only on a very small number of patients with brain disorders, but=20
it shows early promise in two areas.
First, it can help map the normal brain. With a single pulse of the=20
magnet, the magnetic stimulation produces functional brain "lesions,"=20
small regions of tissue that are temporarily paralyzed. Scientists can=20
use these so-called lesions to locate many behaviors, feelings,=20
sensations and mood centers. Second, the technique may also someday=20
help treat certain brain disorders. Researchers using the technique=20
say that with repeated pulses of energy, the magnet literally=20
jump-starts sluggish regions of the brain, much the way electroshock=20
therapy does, restoring normal function for hours to weeks.
Researchers stress, however, that TMS is highly experimental and if=20
used incorrectly can induce brain seizures in healthy people. The=20
technique exploits the natural interplay between magnetism and=20
electricity, said Dr. Mark George, an expert in brain imaging at the=20
Medical University of South Carolina in Charleston. For example,=20
George said, if a steady electric current is run through a coil, it=20
will generate a magnetic field, which can be very powerful. Such=20
electromagnets are used to pick up entire automobiles in scrap-metal=20
yards. When the electric current is turned off, the car falls off the=20
magnet.
Similarly, if a coil of wire is moved through a magnetic field, it=20
will generate a flow of electricity, George said. Such magnetos are=20
used to generate currents for the ignition of internal combustion=20
engines.
Transcranial magnetic stimulation exploits the fact that neurons are
essentially tiny electrical devices. When a nerve cell is activated,=20
it passes a flow of electrons down its length. Upon reaching the nerve=20
end, the electrons induce the release of chemicals that pass to=20
neighboring nerve cells. Thus stimulated, those cells fire an electric=20
current and the process continues, carrying coded messages throughout=20
the brain.
People knew about the electrical nature of nerve cells hundreds of=20
years ago, George said. They thought it would be neat to stimulate=20
such cells externally by applying an electric current directly to the=20
scalp. But the skin and scalp rapidly diffuse directly applied=20
electrical currents, he said. Moreover, it hurts.
Some people also tried putting magnets next to the scalp, George said.=20
But that did not work because the fields were simply too weak to=20
produce an effect. Even if an ordinary magnet were strong enough to=20
penetrate the brain, he said, it would not fluctuate; without some=20
form of motion or change, no electric currents are induced to flow=20
inside the brain.
Nevertheless, there are some who make extraordinary claims that=20
magnets exert healing effects on the brain or psyche, George said.=20
Most experts consider these claims to be totally without merit.
About 10 years ago, British physicists devised a way to get magnetic=20
energy into the brain in a way that generated electric currents inside=20
nerve cells, George said. They ran a strong electric current through a=20
wire coil, turning it on and off every one ten-thousandth of a second,=20
producing a rapidly fluctuating magnetic field that readily passed=20
through the scalp and skull and into the brain.
This changing magnetic field prompts the flow of electrons through=20
nerve cells. Although the nerve cells are stationary, the magnetic=20
fields are changing rapidly enough to naturally induce a current to=20
flow, George said.
The magnets generate fields measuring 1.5 to 2 tesla, which can=20
penetrate the brain by a fraction of an inch. But it is enough to=20
reach the all-important layers of the cerebral cortex, he said.
The magnetic forces are different from those found in familiar=20
magnetic resonance imaging (MRI) machines, which emit strong, steady=20
magnetic fields. Because the fields are not fluctuating, they do not=20
cause brain cells to fire. Furthermore, the fields emitted by electric=20
power lines bear no resemblance to TMS. Those fields are tiny, in=20
comparison, and are no different from the earth's background magnetic=20
field.
In experimenting with the magnetic stimulation technique, researchers=20
have found that they can induce paradoxical effects. A brief, strong=20
magnetic stimulus makes nerve cells fire a rapid train of electrical=20
pulses, George said. Thus activated, the cells no longer accept=20
signals from within the brain. Like a telephone with a busy signal,=20
they are off line, their function blocked temporarily. Take away the=20
magnet, and the cells immediately return to normal.
But if cells are stimulated repeatedly, something else happens, George=20
said. After showing inhibition, they rebound and become more active=20
than before. The effect can last several days or months.
As a first step in probing the effects of TMS, researchers are making=20
brain maps. Different brain regions specialize in specific behaviors=20
and movements. Thus when a single magnetic pulse is applied to a key=20
language area on the left side of the head, people momentarily lose=20
the ability to talk, said Dr. Eric Wassermann, a neurologist and brain=20
imager at the National Institute of Neurological Disorders and Stroke=20
in Bethesda, Md. They are aware of the loss but cannot overcome it=20
until the magnet is turned off.
Similarly, if a magnetic pulse is applied to another part of the head,
people can no longer remember a short list of items recently committed=20
to memory. In this case, they are not aware that they forgot the=20
items.
Three studies have shown that the magnet can affect mood in normal=20
people. When repeated pulses are delivered to an area above the left=20
eyebrow, people feel sad, said Dr. Alvarel Pascual-Leone, a=20
physiologist at the University of Valencia in Spain. One man said he=20
felt the way he did at his grandmother's funeral a month earlier.=20
Others reported feeling apathetic and depleted of energy. But when an=20
area above the right eyebrow was stimulated, subjects said that they=20
felt happier and more energetic. In both cases, the effects wore off=20
after a few hours.
In other experiments, Pascual-Leone is trying to map the acquisition=20
of new motor skills in the human brain. For example, the brain map for=20
the right index finger should expand after a person learns to read the=20
bumps and notches of Braille text. But how quickly does this happen?=20
What happens to adjacent "fingers" in the brain's internal map of the=20
hand?
To find out, Dr. Pascual-Leone recruited novice Braille students and
attached an electrode to the muscle that controlled their right index
fingers. He then laid out a grid on their scalps, over the region
containing the map of the right index finger. A pulse of transcranial
magnetic stimulation over some points in the grid would cause the=20
finger to jump, as the muscle was activated by nerve impulses from the=20
brain. If the map expanded, more points on the grid would elicit this=20
response.
In the experiment, the technique was used before and after class each=20
day for one year. In the first six months of learning, the area of the=20
brain that elicited finger responses expanded dramatically in the=20
hours after class, Pascual-Leone said. But by the next day, it always=20
returned to a smaller, pre-instruction size. After six months,=20
however, the map for the right index finger took on the larger size,=20
whereas maps for adjacent fingers shrank somewhat, Pascual-Leone said.=20
Moreover, the brain's index finger map no longer expanded after=20
Braille class.
"It's not just the finger map that expands" during the six months,
Pascual-Leone said. "You are activating a whole network for Braille
reading. The finger map is a small window into that larger network=20
which has expanded."
Repeated magnetic pulses appear useful in treating phantom limb pain,
Pascual-Leone said, as shown in experiments with people who lost one=20
arm. In amputees who do not experience such pain, the brain map of the=20
stump is larger than the map of the intact limb. But in those who=20
suffer pain, the brain map of the stump has not grown, he said. It is=20
same size as the intact arm region.
"This opens an interesting question," Pascual-Leone said. Would it be
possible to increase the size of the brain map representing the stump?
Repeated magnetic pulses were applied to the brains of 36 amputees
suffering from phantom limb pain. In 31 people, the pain went away for=20
days to months, he said. Experiments are under way to see if more=20
frequent stimulation can permanently alter the maps.
People with various brain disorders may be helped by the technique,=20
George said. For example, clinically depressed patients have decreased=20
metabolic activity in the left frontal lobe and amygdala, two regions=20
involved in planning and emotions. "We don't know what explains it,=20
but when the hypometabolism goes away with drug therapy, talk therapy=20
or electroconvulsive therapy, people get better," George said.
Transcranial magnetic stimulation can be used to jump-start the same=20
brain areas, he said. In experiments at the National Institutes of=20
Health, severely depressed patients got TMS every morning for 20=20
minutes over two weeks. They were later given a sham treatment in=20
which the magnet was held over their head and made clicking sounds but=20
no field was emitted.
The sham treatments had little or no effect on their depression. But
repeated stimulation to the left frontal lobe made many people feel=20
better, George said, while stimulation of the right side had little or=20
no effect. This is opposite to what happens in normal people, he said,=20
possibly because people who are severely depressed have a different=20
brain chemistry or circuitry.
However the technique works, some depressed patients who did not=20
respond to any other treatments said that they felt well for the first=20
time in years, George said. But not everyone responded that way and=20
the effects tended to wear off, he said. Experiments are continuing.
The magnetic technique could someday be an alternative to=20
electroconvulsive therapy, in which a huge electric shock is delivered=20
to the brain under general anesthesia, but it is too soon to tell,=20
Pascual-Leone said. The challenge will be to find the right target in=20
the brain for each person as well as the best dose of magnetic=20
stimulation.
Even then, the technique may only superficially treat brain circuits
involved in depression, said Dr. Harold Sackeim, chief of biological
psychiatry at the New York State Psychiatric Institute. The areas that=20
show low metabolism may be riding on top of permanent deficits in=20
wider brain circuits, he said. Thus magnets may provide temporary=20
relief from depression but would have to be used repeatedly in many=20
people.
Sackeim said he worried that "TMS is getting a lot of hype." Early
experiments are promising, he said, but they have been few in number=20
and not well controlled. "I would be delighted, but very surprised, if=20
the effects of TMS last more than a few days," he said. If there is=20
any benefit, he said, it will come from combining the new technique=20
with drugs and other treatments.
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all the best from janet
on a misty island where it's getting warm enough
for the tree frogs to sing every night
[log in to unmask]
=20
