|Other Abstract||Chronic pain lasts months or years and happens in all parts of the body. It does not only interfere with patients’ daily life, but also can lead to physical and emotional dysfunctions. These problems have negative influences on living and working, which becomes a heavy burden for individuals, families, and the whole society. In practice, chronic pain treatment mainly relies on the use of drugs, which may come along with undesired side effects and adverse consequences, and even lead to potential opioid addiction and overdose deaths. To face such crisis, it is necessary to find safe, effective, nonaddictive interventions and strategies to manage chronic pain.
Vagus nerve system is responsible for visceral movement, visceral sensation, somatic movement and somatic sensation. Transcutaneous auricular vagus nerve stimulation (taVNS), an effective technique to active vagal tone, has already been reported the possibilities of the regulation of pain and psycho-physical functions. However, the mechanisms associated with taVNS intervention are still unclear. Therefore, the main purpose of the present study described in this thesis is to examine the effects of taVNS on physical and emotional functions and pain, as well as the underlying mechanisms through three experiments.
Experiment 1 recruited 17 volunteers and proved that the taVNS with stimulation parameters currently used is capable of inhibiting the shift in cardiac autonomic balance toward sympathetic innervation, suggesting its effectiveness on the vagus nerve system regulation.
Experiment 2 recruited 37 volunteers. Half of them were randomly assigned to the active tVNS group (n = 18), and the rest were assigned to the sham group. Arousal level was selected to reflect physical function. The response time as well as cortical excitability in time frequency domain of the emotional facial dot-probe task were recorded. The results showed that volunteers in the taVNS group had greater reduction of response time than those in the sham-group. Volunteers in the active taVNS group also exhibited a higher change ratio of alpha-ERD at the central-parietal channels after the presentation of the emotional faces, when compared to the sham group volunteers. We concluded that such behavioral and electrophysiological changes reflected the changes of physical arousal caused by active taVNS. In addition, the amplitude of P300 in response to the target dot and/or movement preparation significantly increased at the occipitoparietal electrodes after taVNS.
To investigate the impact of taVNS on pain measurements, Experiment 3 recruited 25 volunteers receiving active taVNS and additional 22 volunteers for sham interventions. Although none significant change of the pain intensity, pain unpleasantness and pain tolerance were observed after both active taVNS and sham interventions, the results showed that active taVNS significantly reduced individual hyperalgesia. However, such positive effect of taVNS on pain threshold cannot be attributed to changes of affective state, given no significantly difference of changes of mood was found between two groups. It implies that there are other physiological or psychological mechanisms responsible for this analgesic effect of taVNS.
In summary, the present study investigated the effects of taVNS on physical and emotional functions and pain. The findings demonstrated that taVNS can significantly increase physical arousal and reduce the hyperalgesia of cold pressor pain. Overall, these results provided reference for future exploration in analgesic mechanism and clinical application of taVNS.|