Single-pulse TMS (monophasic pulses, 100 µs rise time, 1 ms duration) was applied using a Magventure X100 (Magventure, Denmark) and an MC-B35 butterfly coil with 35-mm circular components. Based on feasibility analysis of cross-species TMS comparison70 (link),71 (link), we made use of smaller coils to induce more focal electromagnetic fields to compensate for the small head size of monkeys relative to humans72 (link). The pulse intensity was at 120% of the resting motor threshold, which was defined as the lowest TMS intensity that would elicit visible twitches in at least 5 of 10 consecutive pulses when delivered over the right motor cortex73 (link). For the stability of the TMS setup, a headpost (Crist Instruments) was affixed to the monkey’s skull with screws made of nonmagnetic material. The TMS coil was held in place by an adjustable metal arm. For control purposes, we opted for a sham-condition approach. By this, we rotated the coil 90 degrees over BA46d, thereby ensuring that the sound and vibration (by-products) of the stimulation were identical between the TMS-46d and TMS-sham conditions. Since we have head-posts implanted near the mid-line on the two monkeys, options for control sites (e.g., homologue for the human vertex) were very limited operationally.
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