Transcutaneous Median Nerve Stimulation Protocol for Eliciting Blink Reflex

HBR was elicited by transcutaneous electrical stimulation of the right median nerve at the wrist using a bipolar surface electrode (constant current square-wave pulses; DS7A, Digitimer). First, for each participant, we calibrated the stimulus intensity able to elicit a clear blink reflex by increasing the stimulus intensity until a clear and stable HBR was observed in at least five consecutive trials or the participant refused to increase the intensity further (mean stimulus intensities, 23.35 ± 17.48 mA; range, 0.7–75 mA). Stimulus duration was 200 μs and, to minimize habituation, the inter-stimulus interval was ~ 30 s, as described in the literature^{6 (link)–9 (link),16 (link),51 (link)}. EMG activity was recorded from the right orbicularis oculi muscle (ipsilateral to the stimulated hand), using a pair of bipolar surface electrodes, with the active electrode placed over the mid lower eyelid and the reference electrode placed laterally to the outer canthus. Signals were amplified and digitized at 10 kHz (BIOPAC system, MP150), and stored for off-line analysis.