Effects of single-pulse transcranial magnetic stimulation (TMS) on functional brain activity: a combined event-related TMS and evoked potential study.

OBJECTIVE To further evaluate the potential of slew-rate limiting amplifiers to record electrophysiological signals in spite of concurrent transcranial magnetic stimulation (TMS), and to explore the effects of single-pulse TMS on electroencephalographic (EEG) correlates of functional brain activity. METHODS Visual-evoked potentials (VEPs) to checkerboards were recorded in 7 right-handed subjects, while single-pulse TMS was applied to the occipital pole either at visual stimulus onset, during the build-up or at the expected peak of the early VEP component P1 (VIS&TMS). Timing of TMS was individually adjusted based on each subject's VEP-latency. A condition of TMS without concurrent visual stimulation (TMS(alone)) served for subtraction purposes (VIS&TMS minus TMS(alone)) to partial out TMS-related contaminations of the EEG signal. RESULTS When TMS was applied at visual stimulus onset, VEPs (as calculated by subtraction) perfectly matched control VEPs to visual stimulation alone. TMS at around P1, in contrast, modified the targeted (P1) and the subsequent VEP component (N1), independently of whether TMS was given at build-up or peak. CONCLUSIONS The retrieval of regular VEPs with concomitant TMS at visual stimulus onset suggests that the employed EEG system and subtraction procedure are suited for combined EEG-TMS studies. The VEP changes following TMS at around P1 provide direct clues on the temporal dynamics of TMS pulse effects on functional activity in the human brain. Our data suggest effects of relatively long duration (approximately 100 ms) when TMS is applied while functional neuronal activity evolves.