Can Recordings of Tongue Motor-Evoked Potentials Be Improved? Comparison of Two Electrode Types and Their Reliability

In transcranial magnetic studies (TMS), motor-evoked potentials (MEPs) recorded from the tongue provide important insights into corticobulbar excitability and neuromuscular control. However, their clinical and research utility remains limited due to anatomical and technical challenges, particularly those affecting electrode stability and signal quality.

This study aimed to compare TMS-derived outcomes obtained with traditional surface electrodes and novel suction-based electrodes.

Nineteen healthy adults participated in a repeated-measures, cross-over design. On Day 1, each participant completed two test sessions per electrode type (intra-session reliability). Thirteen participants returned for a third test session at least 1 week later (inter-session reliability). Resting motor threshold (rMT) was determined. MEP amplitude (MEP_Amp) and latency (MEP_Lat) were assessed at five single-pulse stimulation intensities (90%, 100%, 120%, 140% and 160% of rMT). Reliability was analyzed using intraclass correlation coefficients (ICCs, using lower 95% CI bound to gauge reliability strength).

Suction electrodes yielded significantly lower rMT and higher MEP_Amp at 120%-160% rMT. For both electrode types, intra-session reliability for rMT was excellent (ICCs > 0.90) and poor to moderate for MEP_Amp and MEP_Lat (ICCs < 0.75). Inter-session reliability was poor (ICCs < 0.50) across all outcomes for both electrode types.

Compared to surface electrodes, suction electrodes improved signal quality as indicated by lower rMT and higher MEP_Amp. However, both electrode types exhibited substantial intra- and inter-session variability. Although suction electrodes enhance TMS signal quality, the poor test-retest reliability of tongue MEPs currently limits their utility as biomarkers. Methodological refinements are needed to further improve reliability in TMS-based tongue assessments.