Transcranial magnetic stimulation (TMS) is a method that relies on a short-lived magnetic field which is induced by a high current (approx. 5000 Amp) running through a well insulated cable wound into a coil. This field lasts for about half a millisecond and reaches peak amplitudes of 3 Tesla, which is comparable to the field strength used in MRI scanning. Neurons react to these extreme magnetic fluctuations (rising from 0 to 3 Tesla and back to 0 within less than a millisecond) by producing signaling impulses themselves. If the coil, being relatively small and lightweight, is placed to the head of the subject, the nerve cells just underneath the focus of the coil send impulses synchronous to the TMS pulse (normally up to once a second). Since this simultaneous firing of complete neural populations is without any functional content, information processing in this part of the brain is disturbed for fractions of a second. This allows us to induce "virtual lesions", i.e. simulate the failure of the brain region in question without jeopardizing the subject. By deliberately integrating TMS in a well elaborated experiment, one can prove the importance of the brain part in a given cognitive function because performance deficits can be expected. To optimize targeting during TMS experiments, previously obtained (functional) MR imaging data can be utilized for the so called neuro-navigation. A computer compares the brain scan with the subject's head and thus enables a very precise positioning of the coil on the head, just over the brain region of interest.
Magnetic field strength:
0,5 - 3,5 Tesla
Maximum repetition rate:
50 Hz bei 30% max. stimulator output
30 Hz bei 50%
18 Hz bei 80%
15 Hz bei 100%
In our experiments, we normally use stimulation frequencies of 1Hz, and sometimes short pulse series of 10Hz. Duration of a single impluse: 400µs.