Effect of electrode cap on measured cortical motor threshold

We investigated the role of electrode cap use in the determination of the cortical motor threshold (MT), and the resulting changes in the recorded motor evoked potentials (MEPs). We also tested whether the induced changes in determined MT could be corrected via previously introduced correction method. Sixteen healthy subjects were studied. Navigated TMS was used for mapping the optimal representation area of the thenar musculature in the primary motor cortex and individual MTs were determined with and without the use of the electrode cap. A mathematical correction was utilized to compensate for the effect of electrode cap in the MTs. Individual MEPs were also measured. We observed a significant (p < 0.05) increase in the determined MTs attributable to the use of the electrode cap. At the group level this difference was reduced significantly (p < 0.01) by the use of the correction method. However, at the individual level the efficiency of the correction was poor. The MEP-amplitudes were not affected whether measured with or without the electrode cap. The electrode cap affects significantly the cortical MT measured as stimulation intensity making the comparison of MTs difficult with other studies not having used an electrode cap.