TMS is based on the principle of electromagnetic induction; briefly, a changing electric current in the stimulation coil produces a magnetic flux, which in turn induces electric currents in brain tissue. The basic TMS stimulator design involves a capacitive high-voltage, high-current charge-discharge system connected via a switch (usually a thyristor or a silicon-controlled rectifier to prevent ringing in the circuit) to the inductor of the stimulation coil (see Wagner et al, (2007) for further review). The effect of a TMS pulse on cortical activity is dependent on a number of different factors, including the strength of the magnetic flux, the shape of the stimulation coil, the shape and duration of the pulse, the distance and angle between the coil and the cortical surface, the direction of the induced electrical currents, the precise stimulation sequence, and the underlying cortical architecture and activity. One commonly used coil design is the “figure-8” or “butterfly”, in which two round coils are placed side by side such that the currents flow in the same direction at the junction point. As a result, the induced electric
