The tANS (TENS) will be administered using clip electrodes placed on the auricular helix for 30 minutes per session. Stimulation parameters will be: alternating frequency to avoid tolerance; low frequency (2-10 Hz); and high frequency (50-100 Hz). The pulse width will be 125-200 μs, and the current intensity will be individually adjusted to produce a strong but non-painful sensory response.

The tANS (EE) will be administered using clip electrodes placed on the auricular helix for 30 minutes per session. Stimulation parameters will be: biphasic rectangular pulses, with a frequency between 30 and 60 Hz, a pulse width of 200 to 300 μs, a 20-second on / 20-second off duty cycle, and a 2-second ramp-up and ramp-down phase. The current intensity will be individually adjusted to produce a strong but non-painful sensory response.

Trancutaneous Auriculotemporal Nerve Stimulation

The Pressure Pain Threshold (PPT) will be measured through a pressure algometer in 2 different sites bilaterally; 1) temporomandibular joint and 2) thenar eminence. At each site, 3 measurements will be made with an interval of 30 seconds, and the mean will be registered. To determine the PPT, the pressure will be increased at a rate of approximately 1 kg/second and patients will be asked to raise their arm the moment the pressure begins to change to a sensation of pain, at which point, the evaluator will stop pressing. PPT values will be expressed in kg/cm2. The PPT will be determined as the arithmetic mean of the three measurements. A higher score means a better result.

Temporal Summation of Pain (TSP) will be induced using a pressure algometer. For TSP, 10 consecutive pressure pulses will be elicited using the PPT (obtained before). For each pulse of the TSP procedure, the pressure will increase at a rate of 1 kg/second until the PPT is reached, where it will remain for one second before being released. The pressure pulses will be repeatedly performed with an interval between stimuli of one second. Participants will be instructed to assess the pain intensity of the first and tenth pressure pulses according to the numeric pain rating scale (NPRS). The TSP score will be obtained by subtracting the first NPRS score from the last score. The higher the TSP score, the more efficient the nociceptive signaling for the brain.

For Conditioned Pain Modulation (CPM), cold water immersion will be used as the conditioning stimulus (CS) and the PPT as the test stimulus (TS). The participant will immerse their non-dominant hand in a cold water bath at approximately 4°C (±0.5°C) until they reach their maximum pain tolerance or 2 minutes after the start of the immersion, at which point they will remove their hand. The TS will be taken before and immediately after the CS (three PPT measurements). Differences between the PPT values before and after the CS will be considered as CPM values. A greater difference in score indicates more efficient descending inhibition.

Participants will be asked about the presence of adverse effects immediately and 72 hours after the stimulations (e.g., itching or skin irritability at the electrode placement site) and their assessment of which stimulation was best tolerated (using the numerical pain rating scale).