The Effects of Craniofacial Muscle Contractions on the Formerly Vagus Nerve Somatosensory Evoked Potentials

Historical studies reported that electrical stimulation applied over vagus nerve (VN) afferents from the tragus of the human ear-induced skull responses labeled as the vagus nerve somatosensory evoked potential (VSEP). Miscellaneous results acquired from healthy and diseased populations suggested that the origin of the VSEP might not correspond to brain neural activity, but rather to unwanted electromyographic oscillations. Our objective is to definitively demonstrate that scalp recordings labeled as the formerly VSEP (fVSEP) are the expression of muscle activity surrounding recording electrodes.

Using surface electrodes, we electrically stimulated the right ear tragus of five healthy male individuals (mean age: 44 ± 12 years) at 2, 4, 6, 8, and 10 mA, respectively. We recorded the VSEP from the skull of participants while they were relaxed and during controlled voluntary craniofacial muscle movements.

Increasing the stimulus intensity significantly paralleled the increase of the motoneuronal recruitment of the fVSEP during relaxed conditions (eyes open: p = 0.04; eyes closed: p = 0.03). Likewise, voluntary craniofacial muscle contractions significantly modified the duration (p < 0.01) and amplitude (p = 0.013) of the fVSEP.

Our results indicate that the fVSEP is the graphical expression of depolarization and repolarization overflow of signals happening at a distance from the source, as well as the electromyographic expression of unwanted muscle oscillations exerted by craniofacial muscle events registered beyond the point of recording. Therefore, the fVSEP should no longer be considered a brain somatosensory evoked potential nor a measure of autonomic function.