Determining the optimal current direction of transcranial magnetic stimulation to induce motor responses in the tongue: A preliminary study of neurologically healthy individuals

Abstract

Objective: To determine the optimal transcranial magnetic stimulation (TMS) coil direction for inducing motor responses in the tongue in a group of non-neurologically impaired participants. Methods: Single-pulse TMS was delivered using a figure-of-eight Magstim 200 2 TMS coil. Study 1 investigated the effect of eight different TMS coil directions on the motor-evoked potentials elicited in the tongue in eight adults. Study 2 examined active motor threshold levels at optimal TMS coil direction compared to a customarily-used ventral-caudal direction. Study 3 repeated the procedure of Study 1 at five different sites across the tongue motor cortex in one adult. Results: Inter-individual variability in optimal direction was observed, with an optimal range of directions determined for the group. Active motor threshold was reduced when a participant's own optimal TMS coil direction was used compared to the ventral-caudal direction. A restricted range of optimal directions was identified across the five cortical positions tested. Conclusions: There is a need to identify each individual's own optimal TMS coil direction in investigating tongue motor cortex function. A recommended procedure for determining optimal coil direction is described. Significance: Optimized TMS procedures are needed so that TMS can be utilized in determining the underlying neurophysiological basis of various motor speech disorders.

Effects of Coil Orientation on Motor Evoked Potentials From Orbicularis Oris

“…However, while the optimal coil orientation - and resulting induced current direction - for eliciting MEPs from muscles innervated by the corticospinal tract is fairly well-established, recommendations are less clear for facial muscles, innervated through the corticobulbar tract. A handful of studies report on the effect of coil orientation on MEPs elicited for various facial muscles, including Nasalis ( Dubach et al, 2004 ), Masseter ( Guggisberg et al, 2001 ), Depressor Anguli Oris and Depressor Labii Inferioris ( Rodel et al, 1999 ), and muscles in the tongue ( Murdoch et al, 2013 ), but results do not identify a single optimal orientation.…”

Altered Modulation of Silent Period in Tongue Motor Cortex of Persistent Developmental Stuttering in Relation to Stuttering Severity

“…Participants sat in a relaxed position and they were asked to keep their eyes open during stimulations. Murdoch et al [ 54 ] suggested that an optimal coil direction should be individuated in every participant when investigating tongue motor cortex, but this may add experimental variability. As a consequence, after verifying the presence of reliable MEPs, TMS coil was always maintained on the head by the experimenter, normally at 45° with respect to the inter-hemispheric fissure (coil handle pointing backward).…”

Effects of coil orientation on Motor Evoked Potentials from Orbicularis Oris and First Dorsal Interosseous

“…However, while the optimal coil orientation -and resulting induced current direction -for eliciting MEPs from muscles innervated by the corticospinal tract is fairly well-established, recommendations are less clear for facial muscles innervated through the corticobulbar tract. A handful of studies report on the effect of coil orientation on MEPs elicited for various facial muscles, including Nasalis (Dubach et al, 2004), Masseter (Guggisberg et al, 2001), Depressor Anguli Oris and Depressor Labii Inferioris (Rodel et al, 1999), and muscles in the tongue (Murdoch et al, 2013). For instance, Guggisberg et al…”