Instrumentation for the measurement of electric brain responses to transcranial magnetic stimulation

There is described a 60-channel EEG acquisition system designed for the rec ording of scalp-potential distributions starting just 2.5 ms after individu al transcranial magnetic stimulation (TMS) pulses, The amplifier comprises gain-control and sample-and-hold circuits to prevent large artefacts from m agnetically induced voltages in the leads. The maximum amplitude of the sti mulus artefact during the 2.5 ms gating period is 1.7 mu V, and 5 ms after the TMS pulse it is only 0.9 mu V. It is also shown that mechanical forces to the electrodes under the stimulator coil are a potential source of artef acts, even though, with chlorided silver wire and Ag/AgCl-pellet electrodes , the artefact is smaller than 1 mu V. The TMS-compatible multichannel EEG system makes it possible to locate TMS-evoked electric activity in the brai n.