Subjects will receive 10-session BreEStim and 10-session EStim treatment in a random order. The interval between BreEStim and EStim is at least 2 weeks. The following assessment will be made before and after
1. Visual analogue scale (VAS).
2. Quantitative sensory testing (QST). It includes non-invasive assessment of tactile sensation threshold (TST), electrical sensation threshold (EST), electrical pain threshold (EPT), and thermal thresholds.
3. Hear rate variability (HRV). It consists of 5-minute electrocardiogram (ECG) recording before and after a treatment session.