Motor cortex transcranial magnetic stimulation topography assessed by distribution and size of evoked potentials

25 volunteers aged 18 to 70 were examined. Bilateral EMG was performed of m. m. biceps brachii et flexores digitorum profundus using surface pick-up electrodes. The evoked potentials were recorded on Nicolet Pathfinder II. Magstim 200 Novametrix was used for stimulation with A and B sides of a coil with mean diameter of 9 cm. The examinees were sitting on chairs, their forearms flexed voluntarily to approximately 90 degrees, resisting a standing pressure of 5 Nm exerted by the torque motor. Three groups were examined. In the first, we recorded the amplitude, the latencies and spreading of potentials in the muscles on the left and right stimulating with the A and B sides and positioning the coil on the vertex (Cz), 2 and 4 cm forwards and backwards. Moving the coil away from Cz brought about reduction of the amplitudes of the evoked potentials or resulted in the disappearance of potentials. This was better seen in backward shift of the coil. The latencies did not change. The responses were bilateral in 10 out of 11 subjects the amplitudes being higher on the side corresponding to the destination of stimulation. In the second group we wanted to see what the effect of the shift of stimulation away from the meddle line would be. C-P1, C-P2, Cz1 and Cz2 are not suitable sites for obtaining potentials. Stimulation at C-F1 and C-F2 yielded potentials which, contrary to the expectation, evoked ipsilateral responses. In the third group stimulation was combined with focal sensor influence. At the beginning it was a focused attention which, however, was difficult to define.(ABSTRACT TRUNCATED AT 250 WORDS)