(A) A schematic illustrating how electric field coupling interferes with the EEP recording circuit. Here, the loop between one microelectrode and the ground is used as an example. The microelectrode is modeled as a parallel resistor-capacitor for simplification. Note how displacement current (ID), generated by electric field coupling, propagates in both directions once it enters the circuit, while the magnetically induced current (Iind) only propagates in a circular manner. The branch of displacement current (ID1) that flows toward the input end of the amplifier opposes the Iind, counteracting the magnetically induced voltage change across the amplifier input resistance (Rin). The other branch (ID2) flows toward the electrode and can cause polarization at the microelectrode tip. Abbreviations: B, magnetic field; CEL, electrode capacitance; Cin, amplifier input capacitance; Icoil, TMS coil current; REL, electrode resistance. (B) The electrical shield constructed for the Magstim D25 coil. The shield fits tightly with the coil and is grounded through the EEP recording system. (C) Induction waveforms from a pickup probe positioned right below the coil center, along the X-, Y- and Z- axis, with or without the shield, under mspTMS at maximum intensity. Along each axis, the waveforms obtained under shielded and no shield condition overlap, confirming that the shield does not attenuate the magnetic output of the TMS coil. (D) Input voltage to a high impedance buffer (AD825, Vs =±15V), measured with a 1.5 MΩ (1 kHz) microelectrode, and an Ag/AgCl ground electrode in normal saline under mspTMS at maximum intensity with or without the shield. Signal in the ‘Removed’ condition was obtained by taking the difference between the waveforms in ‘No shield’ and ‘Shielded’ condition. The shield restored the correct induction waveform and abolished the voltage offset that leads to the decay.