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 microV, and 5 ms after the TMS pulse it is only 0.9 microV. 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 microV. The TMS-compatible multichannel EEG system makes it possible to locate TMS-evoked electric activity in the brain.
Virtanen
J
J
Department of Psychology, University of Helsinki, Finland. juha@biomag.helsinki.fi
Ruohonen
J
J
Näätänen
R
R
Ilmoniemi
R J
RJ
D004566:Electrodes; D004867:Equipment Design; D006801:Humans; D015225:Magnetoencephalography / Q000295:instrumentation*