Synchronization of neuronal activity in the human primary motor cortex by transcranial magnetic stimulation: An EEG study

Using multichannel electroencephalography (EEG), we investigated temporal d ynamics of the cortical response to transcranial magnetic stimulation (TMS) . TMS was applied over the left primary motor cortex (Ml) of healthy volunt eers, intermixing single suprathreshold pulses with pairs of sub- and supra threshold pulses and simultaneously recording EEG from 60 scalp electrodes. Averaging of EEG data time locked to the onset of TMS pulses yielded a wav eform. consisting of a positive peak (30 ms after the pulse P30), followed by two negative peaks [at 45 (N45) and 100 ms]. Peak-to-peak amplitude of t he P30-N45 waveform was high, ranging from 12 to 70 muV; in most subjects, the N45 potential could be identified in single EEG traces. Spectral analys is revealed that single-pulse TMS induced a brief period of synchronized ac tivity in the beta range (15-30 Hz) in the vicinity of the stimulation site ; again, this oscillatory response was apparent not only in the EEG average s but also in single traces. Both the N45 and the oscillatory response were lower in amplitude in the 12-ms (but not 3-ms) paired-pulse trials, compar ed with the single-pulse trials. These findings are consistent with the pos sibility that TMS applied to MI induces transient synchronization of sponta neous activity of cortical neurons within the 15- to 30-Hz frequency range. As such, they corroborate previous studies of cortical oscillations in the motor cortex and point to the potential of the combined TMS/EEG approach f or further investigations of cortical rhythms in the human brain.