Evaluation of the Cortical Silent Period of the Laryngeal Motor Cortex in Healthy Individuals

Mo Chen, Rebekah L S Summers, George S Goding, Sharyl Samargia, Christy L Ludlow, Cecília N Prudente, Teresa J Kimberley, Mo Chen, Rebekah L S Summers, George S Goding, Sharyl Samargia, Christy L Ludlow, Cecília N Prudente, Teresa J Kimberley

Abstract

Objective: This work aimed to evaluate the cortical silent period (cSP) of the laryngeal motor cortex (LMC) using the bilateral thyroarytenoid (TA) muscles with transcranial magnetic stimulation (TMS). Methods: In 11 healthy participants, fine-wire electromyography (EMG) was used to record bilateral TA muscle responses to single pulse TMS delivered to the LMC in both hemispheres. Peripheral responses to stimulation over the mastoid, where the vagus nerve exits the skull, were collected to verify the central origin of the cortical stimulation responses by comparing the latencies. Results: The cSP duration ranged from 41.7 to 66.4 ms. The peripherally evoked motor-evoked potential (MEP) peak occurred 5-9 ms earlier than the cortical responses (for both sides of TAs: p < 0.0001) with no silent period. The right TA MEP latencies were earlier than the left TA responses for both peripheral and cortical measures (p ≤ 0.0001). Conclusion: These findings demonstrate the feasibility of measuring cSP of LMC based on intrinsic laryngeal muscles responses during vocalization in healthy volunteers. Significance: The technique could be used to study the pathophysiology of neurological disorders that affect TA muscles, such as spasmodic dysphonia. Further, the methodology has application to other muscles of the head and neck not accessible using surface electrodes.

Keywords: TMS; Transcranial magnetic stimulation; cSP; cortical silent period; fine wire electrode; larynx; motor cortex excitability.

Figures

Figure 1
Figure 1
Experimental Set up. (A) Fine-wire electrodes. There were two channels (pairs of fine-wires) inserted into bilateral thyroarytenoid muscles; (B) Ground electrode. The strap ground electrode was placed under the BrainSight subject tracker band.
Figure 2
Figure 2
TMS coil placement for laryngeal motor cortex (LMC) and peripheral stimulation. (A) Coil position and angle for LMC stimulation (cSP test); (B) coil position and angle for peripheral stimulation.
Figure 3
Figure 3
Bilateral thyroarytenoid (TA) muscle responses to right peripheral and cortical stimulation in one participant. (A) Left TA recording without a response to right peripheral stimulation during rest (sample trace); (B) Right TA response to right peripheral stimulation during rest (MEP latency is 5.6 ms.); (C) Left TA recording without a response to right peripheral stimulation during vocalization (sample trace); (D) Right TA response to right peripheral stimulation (MEP peak latency is 5.7 ms.); (E) Average of 50 left TA responses to right cortical stimulation (MEP peak latency is 13.0 ms); (F) Average of 50 right TA responses to right cortical stimulation (MEP peak latency is 11.1 ms). Note the unilateral (ipsilateral) responses to peripheral stimulation and bilateral responses to cortical stimulation, which demonstrates the validity of cortical responses. MEP: motor evoked potential.
Figure 4
Figure 4
Bilateral thyroarytenoid (TA) motor evoked potential (MEP) responses to right cortical stimulation during voice production in Subject 02. (A) 50 individual traces of left TA responses to right cortical stimulation; (B) 50 individual traces of right TA responses to right cortical stimulation; (C) cSP from the left TA during right cortical stimulation (offset X at 42.5 ms); (D) cSP from the right TA during right cortical stimulation (offset X at 49.2 ms). The cSP moving average was calculated based on the average of the 50 trials. The stimuli were delivered at 0 ms. cSP: cortical silent period.
Figure 5
Figure 5
MEP latency comparisons. Red color represents peripheral stimulation responses (left). Blue color represents cortical stimulation responses (right). Thatched represents left TA responses. Solid represents right TA responses. The MEP latencies of left TA were significantly longer than the right TA in both peripheral and cortical stimulation. The MEP latencies of the cortical stimulation were significantly longer than the peripheral one in both left and right TAs. L-H: cortical Left Hemisphere; R-H: cortical Right Hemisphere. *p < 0.05.

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