Within-subject effect of coil-to-cortex distance on cortical electric field threshold and motor evoked potentials in transcranial magnetic stimulation.

Motor threshold (MT) is a global measure of corticospinal excitability in transcranial magnetic stimulation determined over the primary motor cortex. It is commonly quantified as stimulation intensity (SI(MT)) able to induce a muscle response over certain threshold amplitude after 5/10 consecutive stimuli. SI(MT) is known to be dependent on coil-to-cortex distance (CCD). Hypothetically, the effect of CCD on SI(MT) could be removed by using a computed estimate for the stimulus-induced maximum cortical electric field at MT level (EF(MT)). The CCDs of six volunteers were artificially increased by a maximum of 16.3±3.5mm in 5-7 steps. At each CCD, the MT was estimated for the first dorsal interosseous muscle of the right hand as SI(MT) and EF(MT) as well as threshold curves. We found that SI(MT) correlated with CCD while EF(MT) did not. CCD had a significant effect on the within-subject variation in SI(MT) (F(6,28)=80.16, p<0.0001), but not in EF(MT) (F(6,28)=0.69, p=0.656) (analysis of variance). Furthermore, CCD had a minor, but significant within-subject effect on single-trial motor evoked potentials induced at different stimulation intensities, whereas the obvious major effect was caused by stimulation intensity. In conclusion, EF(MT) can be used as a measure of corticospinal excitability instead of SI(MT) to reduce the effect of within-subject variation in CCD.