Medical Examiner

The Body Electric

What is electrical brain stimulation used for? And is it safe?

In an experiment that sounds just slightly like science fiction, researchers at the National Institutes of Health found that by running a small electrical current through the front of the brain, they could markedly improve a person’s verbal agility. Specifically, researchers could increase the number of words beginning with a certain letter that their subjects could list in 90 seconds. Participants who received this intervention, known as DC or direct current polarization, for 20 minutes had measurably greater alliterative facility—that is, they could name 20 percent more words in the time allotted than members of a control group, who had electrodes placed on their scalps but no current applied. Though the research is still in its infancy, DC polarization seems tantalizingly able to allow researchers to target and enhance the functioning of particular brain areas—in this case, regions of the frontal lobe associated with word selection.

DC polarization is but one of several nonpharmacological, noninvasive techniques now generating enormous excitement among neuroscientists. Another, called TMS, or transcranial magnetic stimulation, similarly influences brain circuitry and has been an area of active research since the early 1990s. In TMS, researchers position an electromagnetic coil shaped like a figure eight above the patient’s head and, in an interplay of electric and magnetic phenomena, current running through the coil creates brief but strong magnetic pulses, which pass through the cranium and enter the brain. There, these pulses can induce electrical changes in particular neurons—either increasing or diminishing activity, depending on certain features of the original electrical signal. Undergoing TMS is not painful; subjects feel as though someone is tapping them on the head as the coil causes their scalp muscles to contract rhythmically. They also hear a rapid popping or firing sound from the passage of current through the coil. (In addition to these noninvasive techniques, there are a number of invasive forms of neurostimulation, now widely discussed in the scientific literature. Click here for details.)

For many of us, the mere phrase “electrical brain stimulation” calls to mind highly disturbing images of electroshock therapy, or ECT, in which a very strong current is passed briefly through the brain, causing the patient to have a seizure. Electroshock was once used somewhat liberally to treat depression (it is still used in recalcitrant cases, despite serious side effects that include temporary memory loss); and it has been memorably evoked by a host of harrowing novels and films, including Sylvia Plath’s The Bell Jar, Ken Kesey’s One Flew Over the Cuckoo’s Nest, and the classic film Frances, starring Jessica Lange. The brain stimulation techniques now being researched are, by contrast, generally quite mild. DC polarization, for instance, relies on a much smaller current delivered over a longer period of time, and it appears not to produce side effects, except for a slight itching sensation on the scalp.

So, how exactly do the newer brain stimulation techniques work? How extensive are their potential applications? And are there any downsides?

Most stimulation techniques work because of the brain’s predominantly electrical nature. That is, when a brain cell, or neuron, “fires,” rapid electrical changes occur which allow the cell to signal its neighbors, often passing on a “message” of some kind—the impulse to move a muscle, for instance, or to respond to external stimuli. Interventions like DC polarization and TMS appear to alter the electrical environment in a particular brain area, making it easier (or more difficult) for neurons in that region to fire. As NIH neurologist Eric Wasserman, who led the verbal fluency study, told me, electrodes placed on the scalp can be positioned to create an electric field across a particular brain region; researchers believe that the presence of this field—rather than the current per se—modulates the rate at which neurons fire, thereby revving up the function of a target area.

A limitation of these techniques is that they are unable to activate neurons deep in the brain. But for regions close to the surface, including areas involved in cognition and motor function, both offer an alluringly broad range of potential applications. DC polarization may one day benefit patients who’ve lost some brain function as a result of, say, strokes or frontotemporal dementia, which, after Alzheimer’s, is the most common form of degenerative dementia. Wasserman suggested that DC polarization could likely be used in any disorder where an increase in the firing of nerve cells would be helpful.

Scientists have entertained a somewhat wider array of hypotheses for TMS. Some of the earliest work attempted to use TMS to stimulate areas of the brain’s motor strip, thereby causing particular muscles in the body to contract; researchers were able to position the coil precisely enough over the scalp to cause a twitch in, for instance, their subjects’ index fingers. Such targeted interventions might one day prove useful for patients whose ability to move certain muscles has been compromised by, say, a stroke. TMS has also been explored as a possible treatment for depression—an alternative to electroshock in difficult, drug resistant cases; the results thus far have been mixed but encouraging. In addition, researchers speculate that TMS may be able to boost various kinds of cognitive performance, such as the ability to assimilate information and solve problems. One study has already shown that it can temporarily improve volunteers’ knack for recognizing visual analogies between geometric shapes.

Perhaps most strikingly, in the past few years the military has taken an interest in TMS as a means of improving short-term memory and performance in sleep-deprived soldiers. DARPA, or the Defense Advanced Research Projects Agency, has awarded several grants to researchers who are studying how the brain responds to lack of sleep with the goal of determining whether targeted brain stimulation might be a valuable intervention. One innovation may be the development of high-tech helmets that deliver electromagnetic pulses to exhausted soldiers’ brains, helping them to maintain peak performance, even after days in the field.

To date, neither TMS nor DC polarization has made the leap from laboratory to clinic—neither has been approved by the FDA for the treatment of any particular disorders. At the moment, TMS seems able to stimulate brain regions with greater specificity than DC polarization—it is more “focal.” Yet when used to excite (rather than depress) brain areas, TMS may also carry some risk of inducing seizures. DC polarization appears to be safer in this respect, though there is the possibility that other side effects may yet emerge: The research subject who temporarily becomes a champion wordsmith may find it suddenly impossible to remember where she left her keys. Questions also remain about the consequences of repeated, long-term usage of these techniques.

Still, should research continue to prove promising, it may be that nonpharmacological brain interventions will eventually take their place beside pharmacotherapies, offering another option to patients for whom drugs have proved unhelpful. The potential these therapies offer for performance enhancement may, for healthy individuals, also create the sort of dilemmas that have already emerged with certain drugs, such as Ritalin or antidepressants. Flashing forward five or 10 years, it isn’t too hard to imagine the young neuroscientist secretly zapping herself to solve the great mysteries of the brain—perhaps even finding a cure for writers’ block.