Comparing BOLD Resonses of TMS, Median Nerve Stimulation and Voluntary Finger Movments

V. Kessler, P. Dechent, J. Baudewig 1 MR-Research in Neurology and Psychiatry, Medical Faculty, Georg-August-University, Göttingen, Germany Introduction Transcranial magnetic stimulation (TMS) has developed into a widely used tool in neuroscience to study cortical excitability. However, its underlying mechanisms are still poorly understood. The combination of TMS with functional magnetic resonance imaging (fMRI) provides the possibility to directly visualize the TMS induced changes in brain activity at a high spatial and temporal resolution [1, 2]. The hand area of the primary motor cortex (M1) is known to be an ideal target for TMS because effective stimulation results in hand muscle twitches which can be easily observed or measured. Using interleaved TMS/fMRI we wanted to address the following questions: • Are the activations shown after stimulation with TMS similar to the ones seen after voluntary finger movement and/or peripheral median nerve stimulation? • Does TMS directly stimulate underlying cortical areas? Methods BOLD fMRI was performed at 3 Tesla (Siemens Trio) using EPI (TR 2000ms, TE 36ms, 20 slices, 2x2x4mm3) on 10 right-handed healthy volunteers who were placed in the scanner with the TMS coil over the hand area of M1. Biphasic TMS pulses were applied by a non ferromagnetic figure-of-eight coil connected via an eight meter cable to a Magstim Stimulator outside the magnet room [3]. The individual motor threshold was determined prior to the scanning session. Each study consisted of 5 successive experimental conditions which were randomised between subjects: (1) Subthreshold TMS: TMS at 1 Hz and 90% of the individual motor threshold (2) Suprathreshold TMS: TMS at 1 Hz and 110% of the individual motor threshold (3) Electrical Median Nerve Stimulation (4) Thumb Movement (5) Finger Tapping The conditions (4) and (5) were triggered acoustically by discharging the TMS coil at a low intensity and at the same frequency as during stimulation. The experiment was implemented designed as a block design with epochs of stimulation lasting 12 s and epochs of rest lasting 18 s which were repeated ten times after an initial equilibration period of 18 s. The TMS pulses were applied in the 100 ms gaps between slice acquisitions in order to avoid image disturbances.