To assess the presence of a long-lasting homotopic increase in pain sensitivity to the electrical stimuli after HFS, single electrical test stimuli (square wave pulses with a pulse width of 2 ms) were delivered through the HFS and control electrodes before and after HFS. At each time point (every 5 min), one single electrical pulse was delivered at an intensity of 10 times the electrical detection threshold. The ratings were collected after each stimulus (see pain ratings).

To assess the presence of the heterotopic increase in mechanical pinprick sensitivity after HFS, static mechanical pinprick stimuli (64 and 128 mN) were applied to the skin surrounding both electrodes using calibrated pinprick stimulators (The Pin Prick, MRC Systems GmbH, Heidelberg, Germany). In the original study, seven pinprick intensities (8, 16, 32, 64, 128, 256 and 512 mN) were used. However, because we and others [1135] were able to replicate the increased mechanical pinprick sensitivity after HFS, we decided to use only two pinprick stimulation intensities here (64 and 128 mN), instead of seven. A total of three pinprick stimulations lasting 1 s each were delivered to the skin, with the hand-held stimulator maintained approximately perpendicular to the skin. The mechanical pinprick stimuli were applied within a circular area of 15 mm from the circle of pins of the cathodal electrode and never twice at the same location to avoid sensitization of the skin due to repeated stimulation.

Participants were asked to rate the magnitude of pain elicited by the electrical and mechanical test stimuli on a numerical rating scale (NRS) ranging from 0 (non-painful) to 100 (most intense pain imaginable). This rating scale was identical to the one previously used by [311,35]. Importantly, participants were instructed to distinguish painful from non-painful sensations by the presence of a sharp or slightly pricking or burning sensation. Participants were also instructed to pay attention to any subtle change in the sensation and were free to use integers as well as fractions.

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