Ds7a constant current stimulator

Before starting with the calibration procedure, systolic and diastolic blood pressure was measured to prevent possible hypoarousal reactions caused by basal hypotension. The unconditioned stimulus (US) consisted of a mild electrical shock (train pulse at 50 Hz lasting 200 ms, with a single pulse duration of 1,000 μs) generated with a direct current stimulator (DS7A Constant Current Stimulator, Digitimer). Impulses were delivered through a bar stimulating electrode connected by a Velcro strap on the upper surface of the dominant hand’s index finger. The electrical stimulation intensity was individually calibrated through a staircase procedure (Cornsweet, 1962 (link)), starting with a low current near the perceptible tactile threshold (∼0.5 mA). Participants were asked to rate the aversiveness of each train-pulse on a scale ranging from 0 (not painful at all), 1 (pain threshold) to 10 (highly painful if protracted in time). At the end of the procedure, the US amplitude was set at the current level (mA) corresponding to the mean rating of “7” on the subjective analog scale.

Transcutaneous electrical stimulation (trains of 10 × 1-ms pulses at 333 Hz) was delivered with an isolated DS7A constant current stimulator (Digitimer Ltd., Welwyn Garden City, Hertfordshire, UK) triggered by a Grass S88 train generator (Grass Medical Instruments, Quincy, MA, USA) and controlled by computer with a stimulus presentation program (E-Prime2, Psychology Software Tools, Sharpsburg, PA, USA). Degreased skin over the retromalleolar path of the sural nerve was stimulated by a pair of custom-made surface electrodes (1 cm²; 2 cm inter-electrode distance). The right and left RIII-reflex thresholds were determined separately using the staircase method including 4 series of stimuli of increasing and decreasing intensity, as in our previous studies^{72 (link),73 (link)}. Stimulus intensity was then adjusted individually to 60%, 120% or 140% of RIII-reflex threshold depending on the experimental condition. Pain threshold was also determined using the staircase method and was defined as the lowest stimulus intensity evoking pain. Stimuli were then adjusted individually to 60%, 120% or 140% of pain threshold depending on the experimental condition.

A concentric surface was placed 10 cm from the cubital fossa on the ventral side of the participant’s non-dominant forearm. This concentric electrode (20mm²) activated mainly nociceptive A-delta fibers producing a pinprick-like pain sensation that is typical for these superficial nerve endings in the epidermis^{35 (link)}. Electrical stimuli were provided by a DS7A Constant Current Stimulator (Digitimer Ltd., Welwyn Garden City, Herts, UK). A two-step baseline pain measurement was performed to determine the pain threshold. Electrical stimulation was initiated at 1 mA and was increased by 1 mA for each consecutive stimulus; during a second measure, the stimulation was increased by random increments for each consecutive stimulus. In both measurements, each participant indicated when the stimulus was similar to a pinprick (unpleasant, slightly painful). The stimulus intensity used during the VR conditions was 150% of the average of both pain thresholds, to ensure that the stimuli were painful (mean intensity 15.6 mA ± 5.9). During each study condition, a total of 45 stimuli were given with a random interstimulus interval of five to ten seconds. Subjects were informed that they would receive electrical stimuli but were not informed about the electrical stimulation protocol.

Before starting with the calibration procedure, systolic and diastolic blood pressure was measured in order to prevent possible hypoarousal reactions caused by a basal hypotension. The unconditioned stimulus (US) consisted of a mild electrical shock (train pulse at 50 Hz lasting 200 ms, with a single pulse duration of 1000 µs) generated with a direct current stimulator (DS7A Constant Current Stimulator, Digitimer). Impulses were delivered through a bar stimulating electrode connected by a Velcro strap on the upper surface of the dominant hand’s index finger. The electrical stimulation intensity was individually calibrated through a staircase procedure^{69 (link)}, starting with a low current near the perceptible tactile threshold (~0.5 mA). Participants were asked to rate the aversiveness of each train-pulse on a scale ranging from 0 (not painful at all), 1 (pain threshold) to 10 (highly painful if protracted in time). At the end of the procedure, the US amplitude was set at the current level (mA) corresponding to the mean rating of ‘7’ on the subjective analog scale.

Subjects laid in a supine position with hips and knees flexed at 30°. Two reusable interconnected self-adhering electrodes of 10.2 × 5.1 cm (anode; Uni-Patch™ EP84169, Minnesota, USA) were placed bilaterally on the iliac crests or on the abdominal area based on each subject's comfort level [9 (link), 24 (link), 25 (link)]. The T10 vertebra was identified by palpation of spinal processes and anatomical landmarks. A single self-adhesive electrode (cathode; similar type to the anode) was placed parallel to the spinal processes and covered from T10 to L1-2 vertebral levels. The cathode stimulating electrode over the spinal processes was maintained in place via a Tegaderm transparent film and was held under constant pressure via a custom-made pad throughout the experiment. Both cathode and anode electrodes were connected to a DS7A constant current stimulator (Digitimer Ltd., Hertfordshire, UK). Customized scripts written in Spike 2 triggered the constant current stimulator by single 1 ms duration pulses. We delivered for 40 minutes a total of 480 transspinal single pulses at 0.2 Hz with subjects supine. Transspinal stimulation intensities during the stimulation session ranged from 47.6 to 416 mA across subjects (123.87 ± 25.01) and were equivalent to 1.2 times the TA TEP threshold. At these intensities, the blood pressure remained stable and subjects reported no pain or discomfort.