Mirror-touch synaesthesia: the role of shared ... - UCL Discovery

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Chapter 6
overall duration of roughly 1 ms (Figure 6.1). The extent to which the resulting TMS
pulse disrupts neural processing in the targeted area depends on the orientation of the
coil and the orientation of the underlying nerve fibres (Amassian, Eberle, Maccabee,
and Cracco, 1992). If the induced field is uniform across the cell membrane then no
current will be induced. The TMS effects are optimised when the electric field is
tangential to the orientation of the nerve fibre either due to the electric field
orientation being perpendicular to a straight axon or an axon bending relative to the
orientation of the induced field (Figure 6.2).
Two types of coils are typically used in TMS studies. They are the figure-of-
eight and circular coils (Figure 6.3). All of the studies reported in this thesis use a
figure-of-eight coil, which has been shown to produce the most focal effects of TMS
(Ueno, Tashiro, and Harada, 1988). In the figure-of-eight coil, current flows in
opposite directions around each of the windings and converges on the centre point of
the coil where the electrical currents summate. This leads to focal neural stimulation
with the largest effect occurring in the cortex situated directly under the centre-point
of the coil. Because the outer-windings of the coil are away from the surface of the
scalp they are unlikely to induce an additional disruptive magnetic field. The
stimulation effects dissipate gradually as distance from the maximal point increases
(Figure 6.3).