Functionally navigated transcranial magnetic stimulation to evoke lingual pressure in stroke survivors with dysphagia and healthy adults: a proof of concept trial

Background: Post-stroke dysphagia is characterized by reduced corticolingual excitability and lingual pressure; however, it remains unknown if transcranial magnetic stimulation (TMS) directly facilitates lingual pressure generation.Objectives: To explore optimal procedures for single pulse TMS using neuronavigation to evoke lingual pressure in intact and disrupted neural networks.Methods: Using co-registered functional magnetic resonance imaging, stimulation sites were determined for five healthy adults (Mage = 67) and four stroke survivors with dysphagia and reduced tongue strength (Mage = 66). Evoked lingual pressures were sampled across 45-65% of maximum stimulator output. Healthy participants repeated TMS with a bite block to isolate lingual pressure from off-target stimulation of mandibular elevators.Results: Only one functionally-guided stimulation site fell within previously reported optimal lateral (8-11cm) and anterior (2-4.25cm) coordinates. Lingual pressure was stable prior to pulse and increased linearly with intensity for both groups (p = .005). Post-stroke active motor thresholds were elevated compared to healthy adults (p = .025). Lingual pressure latency remained stable across intensities (p > .05). Jaw stabilization via bite block reduced the mean magnitude of evoked lingual pressure by approximately 16%.Conclusions: Single pulse TMS directly evokes higher lingual pressure and can define motor thresholds in intact and disordered corticolingual pathways. Stimulation sites using neuronavigation in healthy adults and stroke survivors largely differed from external coordinates in the literature that were predominantly established in young adults. Procedures to investigate motor thresholds for lingual pressure generation are proposed. The therapeutic role of TMS to address post-stroke deficits in lingual pressure and corticolingual excitability warrants continued investigation.

Effects of Coil Orientation on Motor Evoked Potentials From Orbicularis Oris

This study aimed to characterize effects of coil orientation on the size of Motor Evoked Potentials (MEPs) from both sides of Orbicularis Oris (OO) and both First Dorsal Interosseous (FDI) muscles, following stimulation to left lip and left hand Primary Motor Cortex. Using a 70 mm figure-of-eight coil, we collected MEPs from eight different orientations while recording from contralateral and ipsilateral OO and FDI using a monophasic pulse delivered at 120% active motor threshold. MEPs from OO were evoked consistently for six orientations for contralateral and ipsilateral sites. Contralateral orientations 0°, 45°, 90°, and 315° were found to best elicit OO MEPs with a likely cortical origin. The largest FDI MEPs were recorded for contralateral 45°, invoking a posterior-anterior (PA) current flow. Orientations traditionally used for FDI were also found to be suitable for eliciting OO MEPs. Individuals vary more in their optimal orientation for OO than for FDI. It is recommended that researchers iteratively probe several orientations when eliciting MEPs from OO. Several orientations likely induced direct activation of facial muscles.

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

Motor balance in developmental stuttering (DS) was investigated with Transcranial Magnetic Stimulation (TMS), with the aim to define novel neural markers of persistent DS in adulthood. Eleven DS adult males were evaluated with TMS on tongue primary motor cortex, compared to 15 matched fluent speakers, in a “state” condition (i.e. stutterers vs. fluent speakers, no overt stuttering). Motor and silent period thresholds (SPT), recruitment curves, and silent period durations were acquired by recording tongue motor evoked potentials. Tongue silent period duration was increased in DS, especially in the left hemisphere (P<0.05; Hedge’s g or Cohen’s d_unbiased = 1.054, i.e. large effect size), suggesting a “state” condition of higher intracortical inhibition in left motor cortex networks. Differences in motor thresholds (different excitatory/inhibitory ratios in DS) were evident, as well as significant differences in SPT. In fluent speakers, the left hemisphere may be marginally more excitable than the right one in motor thresholds at lower muscular activation, while active motor thresholds and SPT were higher in the left hemisphere of DS with respect to the right one, resulting also in a positive correlation with stuttering severity. Pre-TMS electromyography data gave overlapping evidence. Findings suggest the existence of a complex intracortical balance in DS tongue primary motor cortex, with a particular interplay between excitatory and inhibitory mechanisms, also in neural substrates related to silent periods. Findings are discussed with respect to functional and structural impairments in stuttering, and are also proposed as novel neural markers of a stuttering “state” in persistent DS, helping to define more focused treatments (e.g. neuro-modulation).