Mc b35 butterfly coil

We modelled the MagVenture MC-B35 Butterfly Coil (MagVenture A/S, https://www.magventure.com), which is a small figure-of-eight coil (for dimensions, see Table 1), with a set of magnetic dipoles whose locations and weights are those of the integration quadrature points for computing the magnetic flux integral for MEG^{43 (link)}. We built two coil models for the 9-mm-thick coil windings, one with a total of 2568 dipoles in three identical layers (at 1.5, 4.5, and 7.5 mm from the lowermost wire surface) and one with lower resolution, with just one layer (at 4.5 mm) with 136 dipoles. For ease of visualisation of the induced E-field magnitude, we set the rate of change of the coil current to dI/dt = 50 A/µs, which corresponds to about 33% of the maximum stimulator output for MagVenture devices.Table 1

MagVenture MC-B35 Butterfly Coil dimensions according to manufacturer's website (February 24, 2016) and private conversation with the manufacturer's representative.

Inner winding diameter	24 mm
Outer winding diameter	47 mm
Wire thickness	9 mm
Insulation thickness between wires and the coil bottom	2 mm
Number of turns	Two wings, with three layers of four turns each

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 comparison^{70 (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 humans^{72 (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 cortex^{73 (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.