Wassermann says that the preliminary studies are meant to help evaluate how practical the technology is. "We're beginning to think about whether this technology has a role in cognitive enhancement in healthy people--whether it's ethical, whether there is a need and a place for this," he says. Wassermann originally became interested in noninvasive brain stimulation as a treatment for people with neurodegenerative disease, but a series of preliminary tests in patients have been unsuccessful. "It probably won't work in a badly damaged brain," he says. So his team is shifting its attention toward exploring transcranial stimulation as a learning tool in healthy people.

Very little is known about how TDCS works. Scientists theorize that the mild current primes the neurons for action but does not trigger the voltage spikes that neurons use to communicate. "Presumably, it is polarizing neurons and making them more or less likely to respond to inputs," says Warren Grill, a neural engineer at Duke University, in Durham, NC. "But what's happening at the level of the synapse, where the business of learning really takes place, we don't know."

Because the level of stimulation in TDCS is so low, it is considered safer than another noninvasive alternative, transcranial magnetic stimulation. In this approach, which is under investigation as a therapy for stroke and other brain disorders, an electric coil placed over the head generates a magnetic field that passes through the skull, exciting neurons in the brain below. However, because the procedure does trigger neural activity, it carries a risk of seizure.

Cognitive enhancement with drugs such as Ritalin, prescribed for attention deficit disorder, is already widespread, of course. A survey published online at Nature in April found that one in five respondents, most of whom were academics and scientists, reported using such drugs for nonmedical use. Electrical stimulation may prove even easier to access. "Half the people in this room could build this type of device with parts from RadioShack," Wassermann told a crowd at a neurotechnology conference in Cleveland last week.