The study will examine how electrical stimulation of vagus nerve (i.e. nerve around the outer ear) from the skin surface during motor training influences a brain hormone (called norepinephrine), brain activity, and motor performance.

Motor function is compromised with advanced age, and motor impairment is involved in various neuromotor injuries and disorders including stroke, spinal cord injury, amputation, and aging. Development of effective interventions for facilitating neuromotor adaptation is essential for accelerating or augmenting rehabilitation outcomes in the control of impaired limbs. The ultimate goal of the study is to find non-pharmacological and non-invasive neuromodulating interventions for enhancing the rehabilitation outcomes that may be applied to individuals with impaired motor function. In rats, implanted afferent vagus nerve stimulation paired with motor training enhanced neuromotor adaptation and motor recovery most likely through increased release of central neuromodulators that originate from the brainstem. The investigators propose to translate the findings in rats into humans by applying vagus nerve stimulation noninvasively. Transcutaneous VNS (tVNS) can noninvasively activate the brainstem including locus coeruleus, where norepinephrine (i.e. neuromodulator) is synthesized. However, it is unknown whether tVNS leads to increasing neuromodulators and facilitating neuromotor adaptations when combined with motor training in humans. With potential applicability of this novel intervention for facilitating neuromotor adaptation to various clinical human populations in future scope, it is essential to start with the basic understanding about the effect of tVNS on the neuromotor system and training-induced adaptation in neuromotor behavior in non-disabled humans. The overarching hypothesis is that an application of tVNS increases central norepinephrine and facilitates training-induced neuromotor adaptations in humans. The specific aim is to examine the effect of tVNS on central norepinephrine and training-induced neuromotor adaptations in humans. The effect of applying tVNS concurrently to visuomotor training will be investigated by comparing the changes in central norepinephrine and changes in the visuomotor skill and corticospinal excitability due to training with and without tVNS (sham) in non-disabled humans. The visuomotor skill will be assessed with the root-mean-square error of the produced force against the target force, which will be normalized to the maximal voluntary contraction force (MVC) . The investigators expect that subjects with concurrent tVNS during training show greater increases in the visuomotor skill and corticospinal excitability after training. The investigators also expect that tVNS increases central norepinephrine, and the amount of neuromotor adaptations due to training is associated with that of tVNS-induced increase in central norepinephrine. These expected findings will be the first evidence on the efficacy of concurrent tVNS with motor training for upregulating central norepinephrine and facilitating training-induced neuromotor adaptations in humans. They will open new scientific and clinical fields of study that will lead to the creation of motor rehabilitation paired with tVNS that can enhance rehabilitation outcomes in individuals with motor impairment. Demonstration of associated changes between central norepinephrine and neuromotor adaptations due to tVNS in non-disabled humans is a necessary step for applying tVNS to rehabilitation with the understanding of the underlying mechanism and for potentially using central norepinephrine as a predictor of tVNS efficacy in rehabilitation.

Inclusion Criteria:

• Men and women in the age range of 18-39 years will be recruited. All subjects will be healthy and right-handed. Subjects will match the ethnic distribution in the local community.

Exclusion Criteria:

• To ensure the safety associated with TMS and transcutaneous afferent vagus nerve stimulation, following adults will be excluded as in our previous studies (Buharin et al. 2013, 2014) and following the standard recommendations (Keel et al. 2001):

  1. Younger than 18 years old or older than 39 years old
  2. Left-handed
  3. Skilled use of hands (e.g. professional musician)
  4. High blood pressure (>140/90 mmHg)
  5. Had cardiovascular problems
  6. Obese (Body Mass Index: > 30 kg/m2)
  7. Had sensory deficits in your limb
  8. Had alcoholism
  9. Had psychiatric disorders
  10. Had an adverse reaction to TMS (a technique for non-invasive neural stimulation from the brain)
  11. Had a seizure (an abnormal phenomenon of the brain marked by temporary abnormal neuronal activity. Symptoms include involuntary changes in body movement or function, sensation, awareness, or behavior.)
  12. Someone in your family has epilepsy (recurrent seizures marking excessive synchronous neuronal activity in the brain)
  13. Had an EEG (measurement of the electrical activity of the brain through the use of surface electrodes placed on the scalp) for clinical diagnosis
  14. Had a stroke (the loss of brain function due to an interruption in the blood supply to the brain)
  15. Had a head injury (include neurosurgery) that required a visit to a hospital
  16. Suffer from frequent or severe headaches (e.g., migraine headaches within the last six months)
  17. Have any metal permanently in your head (outside the mouth) such as shrapnel, surgical clips, or fragments from welding or metal work. Piercings and other metals on your head are OK if they will be removed before the study.
  18. Have any implanted devices such as cardiac pacemakers (a medical device that uses electrical signals to regulate heart beat), medical pumps, or intra-cardiac lines
  19. Had any other brain-related condition
  20. Had any illness that caused brain injury (i.e. meningitis, aneurysm, brain tumor)
  21. Had severe disease such as cardiologic, pulmonary, renal, endocrinal (hyperthyroidism or hypothyroidism), gastrointestinal or others.
  22. Taking any medications other than over-the-counter medicine
  23. Suspect you might be pregnant (if woman)
  24. Have hearing problems, such as impaired hearing, tinnitus, etc.

Up to one week