EFFECTS OF COIL ORIENTATION AND MAGNETIC-FIELD SHIELD ON TRANSCRANIALMAGNETIC STIMULATION IN CATS

To obtain suitable stimulus conditions for transcranial magnetic stimu lation, the evoked compound muscle action potential (ECMAP), evoked sp inal cord potential (ESCP), and magnetic and electric fields were anal yzed in cats with and without the use of a magnetic field shield. Cats were stimulated using a figure 8 magnetic coil placed on the cranium above the motor cortex. The maximum ECMAP amplitude was recorded when the electric current in the coil was in the mediolateral direction, re gardless of whether a magnetic shield with a 5 x 5 cm window was used. ECMAP and ESCP thresholds were reduced when magnetic shielding was in place. Due to the edge effect, the strengths of the magnetic and elec tric fields were highest in the brainstem area, which is an inhomogene ous volume conductor of the cat's cranium. A large induced electric fi eld directed caudally elicited ECMAP and ESCP responses effectively wh en a magnetic shield with a 5 x 5 cm window was in place. (C) 1998 Joh n Wiley & Sons, Inc.