Transcutaneous electrical nerve stimulation (TENS) is a non-invasive modality that utilizes electrical currents to modulate pain in populations with acute and chronic pain. TENS has been demonstrated to produce hypoalgesic effects in postoperative pain, fibromyalgia, knee osteoarthritis, and healthy subjects. Transcutaneous auricular vagus nerve stimulation (TaVNS) is a non-invasive modality that modulates the vagus nerve by stimulating its auricular branches. The effects of the combination of TENS and TaVNS on producing an analgesic response have not been studied. Considering that TENS and TaVNS both stimulate similar analgesic pathways but through different means of activation, the investigators can hypothesize that a combination of both methods can produce a more pronounced analgesic response. Therefore, the objective of this study is to assess the hypoalgesic effect of a combination of TENS and TaVNS in pain-free subjects.

The study will be a simple crossover design conducted at the University of Hartford. Subjects will be recruited from the University of Hartford population via oral communication, digital flyers, and posters on campus. Thirty participants will undergo two sessions in a crossover manner with one week in between. During one session, the participants will receive TENS with active TaVNS and the other session will be a placebo procedure (TENS with placebo TaVNS). The order of these sessions will be randomized. Importantly, the pressure pain threshold (PPT) and heat pain threshold (HPT) assessors will be blinded to the treatment category. For active TaVNS, a frequency of 25 Hz will be applied with a pulse duration of 200 µs. For placebo TaVNS, the intensity will be increased to a sensory level and then decreased to 0 mA. High frequency TENS of 100Hz will be applied in both sessions, with a pulse duration of 200 µsec, asymmetrical biphasic square waveform, and intensity of maximal tolerance without pain. TENS and TaVNS will be turned on for 30 minutes after a baseline measurement of outcomes. TENS and TaVNS will then be turned off, but the electrodes will remain on until completion of post-treatment assessment. Pressure pain threshold, heat pain threshold, blood pressure, oxygen saturation and heart rate will be tested 4 times: Once pre-intervention, once during intervention, once immediately after the intervention and once 15 minutes post-intervention.

The study will be a simple crossover design with the following two treatments: (1) active TaVNS and (2) placebo TaVNS. Each subject will receive both interventions in random order.

Pressure and Heat pain thresholds will be recorded at baseline, 15 min, 30 min, and 45 min.

Inclusion Criteria:

• Absence of pain
• Age 18-40
• Men and women

Exclusion Criteria:

• Neurological diseases
• Severe cardiorespiratory disease
• Pregnancy
• Skin infection or lesions or change in sensation at the TENS or TaVNS application site
• Cancer
• Cardiac pacemaker
• Allergy to electrodes
• Chronic illness or pain
• Use of drugs that affect pain or vagal tone in the past 48 hours prior to data collection