In recent years, the medical community has been experimenting with what you might call targeted, direct interventions with the brain. Things like drugs will affect any nerve cells that express the protein they interact with, but biology has identified a number of areas of the brain that are associated with specific functions or diseases, and medical researchers are starting to try to manipulate their electrical activity. Later this week, the Proceedings of the National Academies of Science will publish a paper that describes how stimulating the primary motor cortex can help people learn to perform a challenging task involving fine muscle control.

There are a number of ways of performing electrical brain stimulation. In the case of deep brain stimulation, surgeons implant electrodes directly into specific areas of the brain, allowing currents to be sent directly to the relevant cells. This sort of invasive procedure is currently only being tested on crippling diseases such as Parkinson's. Another approach is called transcranial magnetic stimulation, which uses fluctuating magnetic fields to actually trigger the firing of neurons in specific areas of the brain. Since this is far less invasive, it's being tested on a broader range of medical conditions.

The new paper involves an approach that's a step down from those, called transcranial direct current stimulation (tDCS). In this case, electrodes attached to the person's head run a current directly through the brain; the location of the electrodes can target the current to specific areas of the brain. At the cellular level, these currents are extremely weak, but they're thought to reduce the voltage barrier needed for a nerve to fire, essentially enhancing normal activity. Depending on the area of the brain targeted and whether that's closer to the anode or cathode, tDCS has been observed inducing a variety of effects.

The new paper targets an area of the brain known as the primary motor cortex, which helps control muscle movements. The authors focused on a learning task, one they describe as similar in principle to the process we go through when we learn a new sport. Subjects were given a device that measured the pressure applied between the thumb and forefinger, and asked to use it to maneuver a cursor through an on-screen obstacle course. One group of subjects received a current; the controls had electrodes attached, but received no current. The subjects were asked to come in for five consecutive days to repeat the process so that researchers could track how their skill improved.

By the end of day one, those who had an anode placed near the primary motor cortex were already pulling away from their peers (a cathode had no effect), and had opened up a large and significant gap by the end of day five. As expected, stopping the training at day five resulted in a gradual decline of the skills over time. Because the two sets of subjects showed declines of roughly the same rate, the gap that opened up during training wound persisting to at least 85 days after the training sessions ended.

The authors also looked into whether the affect of tDCS comes from the formation of or consolidation of the training by comparing the gains within a day to the retention of skills between days, which they called online and offline training. Overall, the differences in online gains between the two groups weren't statistically significant. Between days, however, the control group was prone to forget some of the skills they had developed; in contrast, those receiving the current actually came back the next day in better shape than they'd left the day before.

The authors argue that this fits in nicely with our model of how memories are formed, as it involves a three-step process of learning, consolidation, and retention. Clearly, the tDCS was only affecting the consolidation portion of the process.

Although the results are startling enough on their own—the fact that something as crude as sticking an electrode on your head is enough to have such specific consequences is quite surprising—they actually have significant practical implications. Strokes and many other types of brain damage often force their victims to relearn basic motor skills, from speech to walking. Given that tDCS is noninvasive and may help speed to recovery of these patients, I'd expect to see tests of its efficacy in the near future.

PNAS, 2009. DOI: 10.1073/pnas.0805413106