Function-guided navigation is commonly used when assessing cortical
excitability using transcranial magnetic stimulation (TMS). However, the
required accuracy, stability and the effect of a change in coil
positioning are not entirely known. This study investigates the accuracy
of function-guided navigation for determining the hotspot. Furthermore,
it evaluates the effect of a change in coil location on the single and
paired pulse excitability measures: motor evoked potential (MEP)
amplitude, TMS evoked potential (TEP) and long intracortical inhibition
(LICI), and of a change in coil orientation on LICI. Eight healthy
subjects participated in the single pulse study, and ten in the paired
pulse study. A robot-guided navigation system was used to ensure
accurate and stable coil positioning at the motor hotspot as determined
using function-guided navigation. In addition, we targeted four
locations at 2 mm and four at 5 mm distance around the initially defined
hotspot, and we increased and decreased the coil orientation by 10°. In
none of the subjects, the largest MEP amplitudes were evoked at the
originally determined hotspot, resulting in a poor accuracy of
function-guided navigation. At the group level, a change in coil
location had no significant effect on the MEP amplitude, TEP, or LICI,
and a change in coil orientation did not significantly affected LICI.
However, at the subject level significant effects on MEP amplitude, TEP,
and LICI were found for changes in coil location or orientation,
although absolute differences were relatively small and did not show a
consistent pattern. This study indicates that a high accuracy in coil
positioning is especially required to measure cortical excitability
reliably in individual subjects using single or paired pulse TMS.

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