Characterization of motor cortical function with navigated transcranial magnetic stimulation

In this Thesis, motor cortical function was mapped with transcranial magnetic stimulation (TMS). Alternative mapping methods were introduced and compared with conventional ones. In Study I, representation areas of excitatory and inhibitory motor responses were investigated. The areas displayed only minor differences and, therefore, inhibitory responses could be mapped instead of excitatory responses. Commonly, TMS is applied as single magnetic pulses. When a pair of TMS pulses is given with specific intensity and timing, motor responses are facilitated and lower TMS intensity is required to induce responses. In Study II, single-pulse and paired-pulse maps did not differ. Hence, motor representations could also be mapped with paired-pulse TMS if single pulses are unable to induce motor responses even with the maximum stimulator output. TMS induces an electric field on the cortex. The spread of the electric field may often result in the overestimation of mapped areas. Therefore, in Study III, the effect of the electric field spread on the motor representation areas was considered by minimum-norm estimation data analysis method.In Study IV, TMS motor maps were compared with those measured with magnetoencephalography (MEG). MEG is also suitable for motor mapping but the MEG maps differed from the TMS maps. In Study V, the optimal inter-train interval (ITI) was investigated for short TMS trains that induce repetition suppression (RS) or habituation of TMS-induced motor responses. Previously applied ITI of 17 s can significantly be shortened to approximately 6 s in order to reduce the measurement duration, while still preserving RS. This Thesis introduces TMS methods that can be used instead of conventional ones, for example, when subjects' cortical excitability is low or when the electric field distribution affects the determination of the representation areas. Moreover, TMS-evoked motor RS may reliably be estimated faster than previously.