What magnetic fields do in the brain -- and how to observe brain activity during transcranial magnetic stimulation

It sounds like science fiction: we can alter the human brain's activity simply by holding a wire coil over the head, resulting in movement of the arms or legs. This technique is called transcranial magnetic stimulation (TMS), and it is much used in research and medicine. In TMS, a strong magnetic pulse induces tiny electrical currents in the affected brain tissue. These currents can activate nerve cells.

In medicine, TMS is used to diagnose impairments of motor function such as in multiple sclerosis or as a result of a stroke. TMS is also used therapeutically, for instance to treat tinnitus, clinical depression, chronic pain or addictions. In Europe, however, TMS is not yet an established method of treatment.

This is partly because researchers still do not really understand what happens on a neuronal level when the magnet is switched on -- even though TMS has been under investigation for more than 30 years. This lack of understanding is due to the fact that neuronal activity in the brain is usually recorded with microelectrodes. But such recordings are massively disturbed by the strong magnetic fields brought to bear in TMS, resulting in masked signals from the neurons' activity.

Now researchers from several groups (Cornelius Schwarz, Martin Giese, Ulf Ziemann and Axel Oeltermann) at three Tübingen institutes (Werner Reichardt Centre for Integrative Neuroscience, Hertie Institute for Clinical Brain Research, and Max Planck Institute for Biological Cybernetics) have cooperatively developed a method to shield microelectrodes against TMS-induced magnetic fields. In this way, they can detect changes in individual brain cells with a delay of only one millisecond after the magnetic pulse.

In their study, the Tübingen researchers show how reliable data can be acquired using their shielding technique. In rats, they stimulated the part of the motor cortex that controls forelimb movement. While the rats moved their forepaws following the magnetic stimulation, the researchers measured their neuronal activity. Observing the cortex neurons' activity directly under TMS, they found that this activity remained for some time after the TMS pulse had ended. Furthermore, the direction of TMS-induced electrical currents in the brain influenced the neuronal activity detected by the researchers. These findings fit with prior experiments done in human subjects, where neuronal activity in the spine and the muscles was measured instead of the brain.

"There are only two research groups in the world who have done something like this," says Dr. Alia Benali, who planned and performed the study. The methods employed by these two groups, how-ever, require exceedingly demanding engineering capabilities. Moreover, they have been developed specifically for primate brains. Because of these restrictions, many laboratories will not be able to make use of these methods. "We wanted to develop a simple method to investigate neuronal activity under TMS. Any given lab should be able to use it without specific know-how," PhD student Bingshuo Li explains.