1. tACS Sham is a temporary non-invasive electrical stimulation that applies a weak oscillatory current to the brain through the scalp for up to 30 second, it is not designed to entrain neuronal activity into any external regulatory frequency patterns.
2. tACS 10Hz is a 10Hz non-invasive electrical stimulation that applies a weak oscillatory current to the brain through the scalp or 20 or 40 minutes to entrain neuronal activity into 10Hz frequency patterns.
3. tACS 20Hz is a 20Hz non-invasive electrical stimulation that applies a weak oscillatory current to the brain through the scalp for 20 or 40 minutes to entrain neuronal activity into 20Hz frequency patterns.

1. tACS 20Hz is a 20Hz non-invasive electrical stimulation that applies a weak oscillatory current to the brain through the scalp for 20 or 40 minutes to entrain neuronal activity into 20Hz frequency patterns.
2. tACS 10Hz is a 10Hz non-invasive electrical stimulation that applies a weak oscillatory current to the brain through the scalp or 20 or 40 minutes to entrain neuronal activity into 10Hz frequency patterns.
3. tACS Sham is a temporary non-invasive electrical stimulation that applies a weak oscillatory current to the brain through the scalp for up to 30 second, it is not designed to entrain neuronal activity into any external regulatory frequency patterns.

1. tACS 10Hz is a 10Hz non-invasive electrical stimulation that applies a weak oscillatory current to the brain through the scalp or 20 or 40 minutes to entrain neuronal activity into 10Hz frequency patterns.
2. tACS 20Hz is a 20Hz non-invasive electrical stimulation that applies a weak oscillatory current to the brain through the scalp for 20 or 40 minutes to entrain neuronal activity into 20Hz frequency patterns.
3. tACS Sham is a temporary non-invasive electrical stimulation that applies a weak oscillatory current to the brain through the scalp for up to 30 second, it is not designed to entrain neuronal activity into any external regulatory frequency patterns.

Sham tACS will applied to the brain through the scalp for 20 minutes.

The visual analog scale (VAS) is a validated, subjective measure for acute and chronic pain. Scores are recorded by making a handwritten mark on a 10-cm line that represents a continuum between "no pain" and "worst pain."

Heart rate variability (HRV) is the fluctuation in the time intervals between adjacent heartbeats (1). HRV indexes neurocardiac function and is generated by heart-brain interactions and dynamic non-linear autonomic nervous system (ANS) processes. HRV will be used as a surrogate assessment for autonomic nerve system