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

There is described a 60-channel EEG acquisition system designed for the recording of scalp-potential distributions starting just 2.5 ms after individual transcranial magnetic stimulation (TMS) pulses, The amplifier comprises gain-control and sample-and-hold circuits to prevent large artefacts from magnetically induced voltages in the leads. The maximum amplitude of the stimulus 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 artefacts, 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 brain.