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 fields add up to a maximum in the region below the junction of the two coils, thereby limiting the area in which the induced currents are sufficient to significantly alter neuronal activity. The precise extent of the cortical surface that is intensely stimulated has been debated, but models and some experimental data on evoked responses suggest that it is on the order of approximately 1 cm2 (Cowey & Walsh, 2000; Wagner et al., 2004).
