Reducing scalp pain for pTES of motor cortex using background hums

Pulsed transcranial electrical stimulation (pTES) with short (< 1ms) high amplitude (> 20 mA) pulses can elicit suprathreshold neural activity and drive physiological effects such as motor evoked potentials. While this technique could benefit several neurological and neuropsychiatric conditions, it is not widely used because the injected current pulses cause substantial pain in the scalp. We investigated approaches to reduce scalp sensation of pTES at motor threshold in human subjects. We introduce the concept of background hums , additional high-frequency, low-amplitude pulse trains that reduce scalp pain. We tested their pain dampening effects, along with varying pTES electrode distance and pulse width. In a subset of 7 participants, we obtained a reduction in pain score of 2/10 compared to a standard pulse. Using these methods, we were able to stimulate above motor threshold two patients affected by fibromyalgia, a chronic pain condition that also heightens pain sensitivity, with a reported scalp pain below 3/10. This work demonstrates that stimulation pain associated with pTES can be actively mitigated, opening the way for clinical applications of pTES.