The Effect of Transcutaneous Vagus Nerve Stimulation on Chinese Lexical Tone Imitation: A Single-Subject Design

Abstract

Background: Transcutaneous vagus nerve stimulation (tVNS) has been reported on its effect to reorganize the sensory neural network in the central nervous system, similar as the observations in the traditional invasive vagus nerve stimulation. Very few studies have examined the impact of tVNS on the plasticity of the motor neural network. This exploratory study was aimed to investigate the effect of tVNS on the motor neural network manifested in laryngeal muscle flexibility and activities during word-level pitch production. Methods: Two volunteers participated in a single-subject design experiment. They were instructed to produce 128 Chinese lexical tones in 16 sessions. tVNS was applied in the second 8 sessions when a stabilized baseline was achieved. Speech samples were collected from CLS 4500b and measured in PRAAT. Results: Among the four types of Chinese lexical tones measured with fundamental frequency in Hz, three have been observed more accurately produced after tVNS application, showing the improved regulation of targeted intrinsic laryngeal muscles. One lexical tone was not improved as its regulation depends on the superior laryngeal muscles that were not targeted. Conclusions: tVNS is a promising non-invasive technique which could modify the brain motor neural network, and it has the potential in facilitating the treatment of motor speech disorders clinically.