Introduction 18 7 8 1 16 21 2 24 24 The aim of this study was to collect normative SSR data. SSRs were measured in both mid-palms following electrical stimulation of the left medial nerve and magnetic stimulation of the neck in healthy controls. SSR onset and peak latency, amplitude and area were analyzed. Electrically generated SSR in the ipsilateral (left) and contralateral (right) palms were compared to the SSR evoked by magnetic stimulation. Methods Subjects Forty healthy adult volunteers [9 males/21 females, mean age 52.21 ± 12.23 (range 28–75) years, height 172 ± 11.3 cm] were studied. All subjects were healthy, with no history of cardiovascular or neurological disease. All subjects denied previous alcohol or drug abuse. None of the subjects had contraindications for magnetic stimulation, such as metal prosthesis or pacemaker. All participants gave written informed consent prior to the start of the study. Procedure 20 Electrical stimulation Single square-wave electrical pulses of 0.1 milliseconds duration were applied to the left median nerve at the wrist at irregular intervals (30–60 seconds) to produce a SSR. The sweep speed was 1,000 milliseconds/D, the amplifiers sensitivity was 100 μV/div and the amplifier filters were set at 0.5 Hz and 2 kHz for the low and high frequency filter, respectively. Five trials were recorded simultaneously from both palms and on each side the response with the largest amplitude and shortest latency were taken. A fixed stimulus intensity of 60 mA was used throughout. Magnetic stimulation A Magstim 200 stimulator, equipped with a circular coil of 9 cm inner diameter and a 2.0-T maximum magnetic field strength, was used to apply magnetic stimulation at the cervical area. The center of the circular coil was placed in the midline over the 7th cervical spinous process. The current was set to anti-clockwise flow in the coil (side A visible) and the stimulus intensity fixed at 60% of maximum output. Five stimuli were delivered randomly every 20–30 seconds. Analysis of waveforms To avoid habituation 5 minutes rest was given between electrical and magnetic stimulation. To maintain uniformity of the results only “P” type responses (waveforms with a larger positive, compared to negative, component) were analyzed. 1 Figure 1 a b  Statistical analysis t P Results Clear SSR recordings with a main positive component (i.e., P waveforms) were obtained in all participants. Latency P 1 2 P P 1 2 Table 1 Normative sympathetic skin response data in healthy subjects  Electrical ipsilateral (L) Electrical contralateral (R) Magnetic (L) Magnetic (R) Latency-onset (seconds) 1.79 ± 0.22 a c d Latency-peak (seconds) 2.54 ± 0.33 a c b,d Amplitude (mV) 2.67 ± 1.49 a 2.74 ± 1.61 2.70 ± 1.69 Area (mV seconds) 2.85 ± 1.53 2.63 ± 1.41 3.24 ± 1.99 d P a b c d Table 2 The difference in sympathetic skin response onset latencies evoked by electrical and magnetic stimulation  Latency difference (milliseconds) Electrical stimulation. R–L 105 ± 85 (0–320)* Magnetic stimulation. R–L 27 ± 86 (−110–330) Left electrical–magnetic 158 ± 183 (−50–640)* Right electrical–magnetic 237 ± 201 (0–780)* P t Amplitude and area During electrical stimulation amplitude was higher of the stimulated (left) side compared to the non-stimulated side. Amplitude was similar during magnetic and contralateral electrical stimulation. The area of the SSR measured at the right side was greater for magnetic stimulation compared to electrical stimulation. Discussion The main finding of this study was that the latency of a SSR depends on whether an electrical or magnetic evoking stimulus is used. We also found a mean latency difference between contralateral and ipsilateral evoked electrical responses of 105 milliseconds. 13 26 2 19 11 13 26 6 3 14 5 14 25 In our study, the latency of the SSR evoked by magnetic stimulation was shorter than the corresponding values of the electrically elicited responses. The shorter latency of the magnetically generated SSR could be caused by impulses bypassing the peripheral afferent part of the reflex. Given that the difference between electrical stimulation at the wrist and magnetic stimulation at the neck was 158 milliseconds and assuming that the average distance from wrist to C7 is 70 cm, the conduction velocity of afferent somatic fibers is calculated to be approximately 5 m/second. 12 17 12 14 4 Based on our results, it can be assumed that the SSR onset latency to spinal cord magnetic stimulation was the shortest, whereas that to contralateral peripheral nerve electrical stimulation was the longest. Furthermore, the difference in latency between SSR to magnetic and ipsilateral electrical stimuli was close to that between SSR to ipsilateral and contralateral electrical stimuli. A satisfactory explanation for the observed differences would not be offered without the risk of being highly speculative. 15 22 23 22 4 10 14 24 24 2 9 3 14 10 14 22 20