Improved recipe for magnetic brain stimulation

Transcranial magnetic stimulation (TMS), in which the brain is stimulated using a magnetic coil held outside the skull, has shown some promise in both studying the brain and in treating mental disorders such as depression, epilepsy, and Parkinson’s disease. Such magnetic fields induce tiny electrical currents inside the skull that alter the activity of neural pathways.

While TMS offers the advantages of relative safety and noninvasiveness, the results of its use in both research and treatment have been disappointing. In human studies, neurological effects of TMS have been transient, rarely lasting longer than 30 minutes.

Now, researchers led by John Rothwell of the Institute of Neurology at University College London have devised a new TMS method that produces rapid, consistent, and controllable changes in the motor cortex of humans that last more than an hour. Their findings offer the potential for both more useful research studies using TMS as well as greater therapeutic application.

In their studies, the researchers applied various patterns of repetitive magnetic pulses to the scalps of volunteer subjects. They aimed the pulses at the motor cortex that controls muscle response, because effects on the motor cortex can be objectively measured by recording the amount of electrical muscle response to stimulation. Specifically, the researchers positioned the magnetic coil over the motor cortex area that controls hand movement, and they measured response by determining the amount of muscle response in a small muscle in the subjects’ hands.

The researchers recognized the ethical issue of experimenting on healthy human subjects who had nothing to gain from such experiments. So, they began their studies with stimulations of smaller intensities and lower frequencies than they ultimately used in the experiments. Those initial tests showed that there were no long-lasting or side effects from such stimulations.

In their experiments, the researchers found that they could produce controllable, consistent, and long-lasting effects using short bursts of low-intensity pulses over a period of 20 to 190 seconds. Significantly, the researchers found they could overcome the shortcomings of previous stimulation approaches that produced a mix of both excitation and inhibition of transmission of signals between neurons in the brain. The researchers discovered that the excitatory effect of TMS builds up rapidly, within about a second, while the inhibitory effect builds up within several seconds. Thus, by adjusting the length of stimulation, they could favor excitatory or suppressive effects on the brain.

“We have found these stimulation paradigms to be safe in normal subjects and capable of producing consistent, rapid, and controllable electrophysiological and behavioral changes in the function of the human motor system that outlast the period of stimulation by more than 60 minutes” concluded the scientists.

“The method may prove useful not only in the motor cortex but also in other regions of the brain for both the study of normal human physiology and for therapeutic manipulation of brain plasticity,” they concluded.