A simple, inexpensive technique under investigation for stimulating neurons may promote recovery from stroke and other brain trauma and could enhance normal brain function, according to the authors of a study published Oct. 28 in BMC Neuroscience.
Transcranial direct current stimulation, or tDCS, consists of sending mild direct electrical current between two electrodes held against the scalp. A small amount of the current passes through the skull. Anodal, or positively charged current, increases the excitability of neurons underneath by up to 40 percent within minutes. This effect may last for 30 to 90 minutes after a 20-minute to 30-minute stimulation session, and longer with repeated sessions. Cathodal, or negatively charged current has the opposite effect—it inhibits the activity of underlying brain tissue.
The researchers applied tDCS from a constant current stimulator powered by a 9-volt battery to electrodes applied above each ear to both sides of the motor cortex of right-handed people asked to perform a sequence of keystrokes with the four fingers on their left hand.
As expected, applying anodal tDCS to the motor cortex controlling the fingers of the non-dominant hand improved their performance by 16 percent. But when the researchers simultaneously applied cathodal current to the motor cortex of the other hemisphere, which inhibited the neurons controlling the dominant right hand, the subjects’ performance with their non-dominant left hand improved by 24 percent.
Amplifying the effect of tDCS in this way could help people learn new motor skills and recover motor skills lost due to stroke, says Gottfried Schlaug of Harvard Medical School, the lead author of the study.
“The ultimate goal is to use tDCS in stroke recovery,” he says, “but it also could be used to promote motor skill acquisition and learning in general. For example, having a technique that could facilitate the acquisition of motor skills could be beneficial for individuals who need to develop fine motor skills, such as instrumental musicians.”
Eric Wassermann, a neurologist and neuroscientist at the National Institute for Neurological Disorders and Stroke who has done his own experiments with tDCS, was impressed with the use of anodal and cathodal current simultaneously to stimulate neurons on one side of the brain while inhibiting corresponding neurons in the other hemisphere.
“The sort of push-pull thing these folks have done is interesting,” Wassermann says. “I see a bright future for this. It has a lot of promise as a therapeutic modality, and possibly as a way of enhancing the capacities of healthy people”
Leonardo Cohen, a neurologist and chief of the Human Cortical Physiology Section of the National Institute of Neurological Disorders and Stroke, and other researchers have applied the direct-current technique to stroke patients. Although Cohen considers the work preliminary, he believes tDCS could enhance rehabilitation efforts for stroke patients, helping them regain the ability to perform activities of daily living such as feeding and dressing themselves.
Unlike a similar technique known as transcranial magnetic stimulation (TMS), which can cause resting neurons to discharge, tDCS is a gentler technique that merely increases or decreases the activity of neurons that are already firing.
“It (tDCS) is very benign in its action,” Schlaug says. “TDCS has never been associated with seizures or adverse side effects.” Wasserman, however, cautioned that too much current could cause skin burns. “Nobody has done a systematic study of what produces this effect,” he said.
Although discovered in the 1960s, tDCS was largely forgotten until recently, when researchers started finding new potential uses for it. For example, subjects in a recent experiment who received anodal tDCS to their motor cortex scored better on a test that involved rapidly pressing keys in response to computer instructions. In another experiment, subjects who received anodal tDCS to the left prefrontal cortex deciphered the rules of a computer game faster than control subjects.