Electrify your mind - literally
- 15
April 2006
-
Bijal
Trivedi
Testing tDCS
against dementia
(Image: Marc Asnin/New
Scientist)
LINDA BUSTEED sits nervously as two
electrodes wrapped in large, wet sponges are strapped to her head. One
electrode grazes the hairline above her left eye while the other sits squarely
on her right eyebrow. Wires snake over her head to a small power pack fuelled
by a 9-volt battery. Busteed drums her fingers on the
table as she anticipates the moment when an electric current will start flowing
through her brain.
It sounds like quackery, but it's not. A
growing body of evidence suggests that passing a small electric current through
your head can have a profound effect on the way your brain works. Called
transcranial direct current stimulation (tDCS), the
technique has already been shown to boost verbal and motor skills and to
improve learning and memory in healthy people - making fully-functioning brains
work even better. It is also showing promise as a
therapy to cure migraine and speed recovery after a stroke, and may extract
more from the withering brains of people with dementia. Some researchers think
the technique will eventually yield a commercial device that healthy people
could use to boost their brain function at the flick of a switch.
“You could use this to boost
your brainpower at the flick of a switch”
Busteed isn't here to test commercial devices, however. The
64-year-old suffers from the degenerative brain disease frontotemporal
dementia, which leads to language loss, personality changes and mood swings.
There is no treatment.
Busteed is one of 20 patients in a phase II clinical trial
led by Eric Wassermann, head of the brain stimulation unit at the US National
Institute of Neurological Disorders and Stroke (NINDS) in Bethesda, Maryland.
He wants to know whether a 40-minute burst of direct current directed at her
left frontal lobe can improve her ability to generate lists of words, a
hallmark deficit of her disease. Wassermann's study is double-blind, so he
won't know whether Busteed is receiving current or
not. Busteed probably won't know either - tDCS is silent and elicits barely a tingle. If she is
getting the real thing, Wassermann hopes that the current will "squeeze
more out of the sick neurons", enabling Busteed
to perform better.
If the trial proves successful, Wassermann
would like to develop a brain stimulation device that patients can take home
and use whenever they want. He envisages a gizmo about the size of an MP3
player, perhaps incorporated into a hat. "Turn it on and you feel
better," he says. "Turn it off and you're back where you
started." It sounds too simple to be feasible, but studies from around the
world suggest that Wassermann has a good chance of success. "All the
scientific literature points in the same direction," says neurologist
Leonardo Cohen, chief of the stroke and neurorehabilitation
clinic at NINDS. "There must be something to it."
Zapping the brain with electricity to cure
various maladies has slipped in and out of vogue over the past two millennia
(see "Zaps from the past"). In recent years, however, it has fallen
out of favour, superseded by a more powerful non-invasive
technique called transcranial magnetic stimulation. TMS works by penetrating
the skull not with electricity but with a magnetic field, causing all the
neurons in a particular region to fire in concert. After TMS stimulation stops,
depending on the frequency of magnetic pulses, this can have the effect of
either switching that region on, or turning it off.
TMS has proved exceptionally useful for
mapping brain functions and has also been tested as a therapy, but it can be
unpredictable and dangerous. Neurons in the brain normally fire asynchronously
as they communicate, but TMS can produce a massive synchrony of activity that
can propagate through the cortex like a Mexican wave through a stadium. If this
happens brain activity shuts down momentarily and
causes seizures. Despite an established safety margin for TMS, there is always
a remote possibility of triggering a seizure, which means that any treatments
have to be monitored by a physician. The bulky nature of the device also makes
it difficult to use outside a hospital.
The rediscovery of electrical stimulation
began in 1999, when neurologists Walter Paulus and
Michael Nitsche of the University of Göttingen in Germany attended a conference at which they
heard about an experimental technique combining TMS with direct current
stimulation. They went back to their lab intending to try it for themselves,
starting with electricity alone. Those first results were "so amazing and
encouraging", says Paulus, that they wanted to
know more.
In that first experiment, Paulus
and Nitsche took a group of healthy volunteers and
stimulated their motor cortices with direct current. They found that tDCS increased the neuronal firing rate by up to 40 per
cent. Where the effect differed from TMS was that it only affected neurons that
were already active - it didn't cause resting neurons to start firing. They
also discovered that if they applied tDCS for 3
minutes or more, the effect lingered after the current was switched off,
sometimes lasting for several hours. The experiment suggested that tDCS was safe, painless and non-invasive and that the
effects on neuronal excitability could potentially have a profound, if
temporary, effect on brain function.
Wassermann was intrigued by the impact of tDCS on healthy brains and began laying the groundwork for
his own trials. In the past five years, he, the Göttingen
team and others have been testing the potential of tDCS,
primarily for the brains of healthy volunteers but increasingly as a therapy
too.
Administering tDCS
is relatively easy. It is essentially a matter of strapping two electrodes to
your head, positioning them, adjusting the current to between 1 and 2 milliamps
and choosing the right duration.
The current is very weak and most people feel
nothing, except in some cases a "slight tingle or itch", says
Wassermann. The human head is a poor conductor, he adds, estimating that at
least 50 per cent of the current is lost, shunted across the skin as it follows
the path of least resistance to the other electrode. But measurements of neural
activity prove that some current does pass through the brain.
What exactly is happening is unknown, but
experiments with humans and animals, as well as recordings from individual
neurons, suggest that it can either increase the activity of neurons that are
already firing, or damp it down, depending on the direction of the current and
how the neurons are aligned.
Neurons in the cerebral cortex tend to be
arranged with their information-gathering dendrites pointing outwards, towards
the scalp, and their information-transmitting axons projecting inwards. When
the positively charged tDCS electrode is close to the
dendrites, the current causes active neurons to fire more frequently. The
negative electrode does the opposite. So if you know the region of the cortex
you want to target, you can zap it with one of the electrodes to either
stimulate it or inhibit it. Of course, the area under the second electrode is
experiencing the opposite effect. "This bothers me to no end," admits
Wassermann. But he says that if you place the second electrode just above an
eye, it is distanced from the brain by bone and sinus.
The overall effect of tDCS,
says Cohen, is to make the excited area work more effectively. "It's like
giving a small cup of coffee to a relatively focal part of your brain - the one
that you know will be engaged in the performance of certain tasks," he
says. "The one you need to do the task better."
So far so good, but does this trickle of
charge have any effect on cognitive performance? In 2003, Paulus's
team produced evidence that it does (Journal of Cognitive Neuroscience, vol 15, p 619).
The researchers asked volunteers to press
keys in response to instructions on the computer screen. What the volunteers
didn't know was that the sequence of keystrokes followed a subtle but
predictable pattern. With stimulatory tDCS applied to
their primary motor cortices, the volunteers learned the sequence significantly
faster than normal. Stimulating different brain areas or applying inhibitory or
"sham" tDCS had no effect.
Paulus and colleagues have since gone on to produce more
positive results. Plying the left prefrontal cortex with stimulatory tDCS, for example, boosts performance on a different test
of learning and memory. They showed volunteers combinations of squares,
circles, triangles and diamonds and asked them to guess whether that
combination was "sunny" or "rainy". At first the task is
baffling, but eventually, by trial and error, volunteers discover hidden rules
and start scoring higher than chance. According to the researchers, volunteers
who received tDCS stimulation got the gist
significantly faster.
It's not just stimulatory tDCS
that can give your brain a boost. Last year Andrea Antal,
a member of Paulus's team, reported that inhibitory tDCS can work too. She used tDCS
to inhibit activity in a region of the visual cortex called V5, which helps
perceive movement. The result was improved performance on a visual tracking
task in which the subject had to follow a dot on the computer screen that could
come from one of four directions.
"At first we were utterly surprised that
inhibitory tDCS makes something better - it should be
worse," says Antal. However, she says, the task
is very complicated and produces a lot of neural activation and noise. Perhaps tDCS improves the signal to noise ratio.
The Göttingen team
isn't the only one with success stories. Last year researchers at Beth Israel
Deaconess Medical Center in Boston, Massachusetts, showed that working memory,
the sort used to memorise facts or lists of words,
can be improved with stimulatory tDCS. "It's a
bit like increasing the amount of RAM available," says team leader Alvaro Pascual-Leone.
Wassermann himself tested tDCS
on the left prefrontal cortex of 103 volunteers and saw a 20 per cent
improvement in their ability to generate lists of words beginning with a given
letter. A handful of people even noticed the difference. "They didn't say
'I feel like superman', but they did notice that they were performing
better," says Wassermann. Taken together, he says, these results suggest
that tDCS really can be used to boost brainpower
beyond its normal limits.
It is also showing promise as a therapy. Antal is testing inhibitory tDCS
for migraine and the associated sensations of flashing lights, strange colours and blurred vision, known as auras. She says that
while tDCS does not work for all types of migraine,
in many people it reduces pain and stops the auras.
Cohen, meanwhile, has tested the technique on
stroke patients. He stresses that he has tried it on less than 40 people so
far, and that up to now the results are only proof of principle. Still, from
what he has seen he thinks that tDCS in combination
with rehab could help some patients regain movements that would help them do
things such as eat, turn pages and grasp small objects. "The most
important point is that the magnitude of improvements correlates with increases
in the excitability of neurons," he says. "This suggests cause and
effect."
Overall, it seems that tDCS
has real promise, though many questions remain. Key among those is the full
range of brain functions that could be enhanced. Wassermann speculates that
almost any brain function associated with a specific, localised
region of the cerebral cortex is potentially amenable to tDCS.
Anything buried deeper in the brain, however, is probably not accessible except
via dangerously strong currents.
Independent experts are somewhat divided.
"Whether low DC current can produce cognitive effects is an open question
but I wouldn't rule it out," says Ralph Hoffman, professor of psychiatry
at Yale University. "The physiology is plausible. It doesn't sound
nutty." Dominique Durand, director of the neural engineering centre at
Case Western Reserve University in Cleveland, Ohio, is less impressed. "I
think it is pushing it because this is not selective," he says. "It
basically stimulates a large part of the brain."
The biggest unknown, however, is whether tDCS will be more than a flash in the pan. "What we
are most concerned about is that it will work a couple of times and then won't
work again," says Wassermann. Just as you can become habituated to a
strong smell if you are exposed to it for a long time, it is possible that a
brain region exposed to a direct current more than once or twice in a short
space of time will get used to it. If habituation does occur, says Wassermann,
the technique is useless. "If this can't do something for somebody then
forget it. It just becomes a funny phenomenon."
Wassermann and other researchers, however,
are satisfied that at the very least tDCS is safe.
What is more, the device itself is tantalisingly
simple and would be cheap and easy to make. "It's comfortable, easy and
inexpensive, and it seems to work," says Cohen. Adds Wassermann:
"Anyone with the know-how could go to an electronics store, buy the
components and build one." If tDCS proves its
worth, he is interested in developing a commercial device. He points out that
you can already buy headgear that claims to cure insomnia, anxiety and
depression by stimulating your brain with alternating current, even though
there is scant evidence that it works. Imagine the potential for a brain
stimulator that really does the business.
So if the day comes when you can buy a
battery-powered thinking cap, what use might it be? One possibility is that it
could help you learn new, improved skills. The results with motor learning and
visual tracking, for example, might translate into a better tennis game or
improved piano playing. "And if you can enhance motor learning with tDCS then it might help you learn something else,"
agrees Wassermann. It's conceivable that enhanced learning and verbal skills
could make it easier to learn a second language or expand your vocabulary, says
Cohen. Students might even be able to raise their game by giving themselves a
blast of tDCS before class.
Another possibility, says Wassermann, is
using tDCS to boost your alertness. Researchers
funded by the US military have already expressed interest in developing that
side of the technology for pilots (New Scientist, 18 February, p 34).
"Fighter pilots land on aircraft carriers at the worst times of night
after working long hours," says Wassermann. "Suppose you have this
device in your helmet, you could flick it on before landing and get much more
alertness."
It sounds too good to be true, and it may
turn out to be. But if tDCS lives up to its promise
perhaps all you'll need to boost your brainpower is a 9-volt battery, a couple
of wires and some pieces of wet sponge. Now there's an electrifying thought.
From issue 2547 of New
Scientist magazine, 15 April 2006, page 34
Psychiatry's Shocking New Tools
By Samuel K. Moore
Psychiatrists
are beginning to look at an even simpler technology than transcranial magnetic
stimulation to fight depression. "It's like hooking the patient up to a
car battery," jokes Sachdev. "But with
safety features," his colleague Colleen Loo, a
senior research fellow, hastily adds. Crude or not, it's a pretty accurate
description of an experimental technique called, or tDCS.
Basically, it subjects the front half of the brain to a minutes-long 1-mA
direct current once a day for several weeks "
|
|
TECHNICAL
ILLUSTRATION: BRYAN CHRISTIE
Transcranial Direct Current Stimulation: A device
drives a small direct current through the front part of a patient's brain.
Though the stimulation is done only for minutes a day over a period of weeks,
it appears to alter the activity of neurons in the long term.
The
simplicity of tDCS makes it sound almost suspicious,
and indeed its origins stretch back into the murk of 19th-century quackery. But
the principle of how tDCS seems to work in the brain
is roughly the same as that of rTMS. They both seek
to make neurons in the prefrontal cortex, the decision-making part of the
brain, more excitable, that is, more likely to propagate a signal from neuron
to neuron. In tDCS's case a small current, delivered
via electrodes on the temples, biases brain cells, making them more likely to
emit a spike of voltage, says Alvaro Pascual-Leone,
associate professor of neurology studying tDCS at
Harvard University, in Cambridge, Mass. The effect, studies have shown, lasts
long after the current is turned off.
The
concept and technology are so simple, in fact, that Pascual-Leone
and his colleagues suggested in The British Journal of Psychiatry that tDCS be used in the developing world as a first-line
treatment for depression instead of rather expensive antidepressant drugs. But Sachdev thinks this is a terrible idea. "We need to
know a lot more about tDCS before it is accepted as
an effective treatment and must await the results of many ongoing trials,"
he wrote in a rebuttal. "In the meantime, depressed patients in the
developing world should be dissuaded from unplugging their car batteries and
clamping them on their foreheads."
Pascual-Leone says he has results showing tDCS fought treatment-resistant depression as well as rTMS did in experiments done at the University of São Paulo School of Medicine, in Brazil, but at press
time the study had not yet been published in a peer-reviewed journal....
Direct activation of a corticospinal
axon with transcranial electrical stimulation
 |
The
animation shows the activation of a corticospinal axon
with transcranial electrical stimulation. The
membrane potential is illustrated with colors: hot colors show the action
potential. The time span of the animation is 600 microseconds. Stimulus is
illustrated with the red and blue color of the electrodes. The
axon is activated directly at a deep bend at a location corresponding to the
fiber entering the midbrain. The propagating action potential initiated nearer
surface is blocked due collision. See
details in 'Veikko Suihko (1998): Modeling
direct activation of corticospinal axons using transcranial electrical stimulation.
Electroencephalography
and clinical Neurophysiology, 109, 238-244.'
|
Doctor
Electric: A Handy Electromagnetic Gadget Stimulates the Brain
Feeling foggy? Take 2 milliamps to the brain and call me in the
morning.
by
Vanessa
Richardson
Credit:
Hugh D'Andrade
Imagine
you have a headache. Or you're too fried to face a busy workload. Rather than
taking aspirin or coffee, you put a small device the size of an iPod to the
back of your head and push a button to revitalize your brain. Sound futuristic?
It may not be many years away.
Scientists
have scrutinized the human brain for thousands of years, but with the use of
electricity and magnets, medical researchers are getting closer to identifying
the areas -- and creating the tools -- that stimulate and repair the mysterious
organ.
Eric
Wassermann, a neurologist and chief of the Brain Stimulation Unit at the National
Institute of Neurological Disorders and Stroke (NINDS), has come closest to
creating an inexpensive, painless "thinking cap." The device runs on
electrical currents, known as transcranial direct
current stimulation. His studies have shown that tDCS
can boost verbal skills in healthy people by as much as 20 percent.
In
one study, volunteers were asked to recall and say as many words that begin
with a particular letter as possible, then passed a tiny (2-milliamp) current
through electrodes attached to their foreheads. The volunteers were quizzed
again using a different letter with the current on and were able to come up
with 20 percent more words. The only side effect so far has been itching or
tingling on the scalp.
Wassermann
can't pinpoint exactly what is happening, but he thinks tDCS
lets the prefrontal cortex, the brain part associated with verbal memory,
transmit signals more easily. Any function associated with a specific region of
the cerebral cortex (the outer edges of the brain) is potentially within tDCS's reach. The goal is to make the targeted area work
more effectively, like giving it a small cup of coffee.
"It
doesn't cause neurons to fire on their own -- it needs to have some drive on
them to do so," says Wassermann. "It's a little like treating
specific nerve cells locally with a drug, so it could be a very helpful way of
boosting brain function in people with brain disorders and injuries."
Even
though he is focusing on tDCS for more heavy-duty
problems like head injuries and dementia, Wassermann does not rule out a
thinking cap that any healthy person could use to boost brainpower with the
flick of a switch. The device is already simple and easy to make, he says.
"Anyone with the know-how could go to an electronics store, buy the
components, and build one. It's simply a 9-volt battery, a couple of wires, and
some pieces of wet sponge. The question now is, what part of the brain do you
stimulate, and how can it actually help you? That's what we're still trying to
learn."
Neurologists
are also using electricity to try to treat various brain disorders, from
Parkinson's disease to headaches. As early as 45 BC, a Roman court physician
named Scribonius Largus
noted that the application of live torpedo fish, a type of electric ray, to
patients' foreheads cured headaches. Greek physician and philosopher Galen
noted the same findings a century later.
The
modern version of Largus's and Galen's fish involves
magnetic pulses in an up-and-coming treatment known as transcranial
magnetic stimulation. Techniques are still being refined, but researchers know
that by placing a TMS device on different areas of the head, they can make
fingers twitch or freeze speech in mid-sentence. Doctors treating depression
aim the magnets at the prefrontal cortex, while those treating migraine
headaches go toward the nerve centers in the back of the head.
"There's
evidence that migraines start with electrical hyperexcitability
in the brain's cortex," says Yousef Mohammad, a
neurologist at the Ohio State University Medical Center, who has found evidence
that a TMS device placed against the back of the head can prevent migraine
pain. "Our theory is that if we can break that with two pulses of an
electromagnetic field, we can abort a headache before it starts." Mohammad
is working with a medical company to research a portable TMS device the size of
a hair dryer, and recently launched a bigger study nationwide.
Wassermann
has also tested TMS by using himself as a guinea pig. He had a fellow
researcher target his brain's speech centers by zapping him while speaking,
which stopped him in mid-sentence -- a feeling he calls
"indescribable."
So
next time you feel frazzled or foggy, think of strapping on a catchy-looking
gadget and gently jolting your way to rejuvenation. Now, that's an electrifying
thought.
Vanessa Richardson is a freelance writer in San
Francisco.
This
article was also published in the November 2007 issue of Edutopia magazine.
To Probe
Further
For
a more complete roundup of the clinical research into the new device-based
therapies, see Brain Stimulation in Psychiatric Treatment,
edited by Sarah H. Lisanby, Washington, D.C.,
American Psychiatric Publishing (2004).
A
Neuronetics executive teaches you how to design a transcranial magnetic stimulator in "Designing Transcranial Magnetic Stimulation Systems," by K.
Davey and M. Riehl, IEEE Transactions on Magnetics,
March 2005, pp. 1142–48.
More
details of vagus nerve stimulators are laid out in
"Vagus Nerve Stimulation for the Treatment of
Depression," by Dorin Panescu,
IEEE Engineering in Medicine and Biology Magazine, November–December 2005, pp.
68–72.
Links:
http://www.newscientist.com/data/av/podcast/newsci-20060414-electrify.mp3
