Electroencephalography and clinical Neurophysiology, 1988, 69:585-588 585 Elsevier Scientific Publishers Ireland, Ltd. EEG 03505 Short communication Recovery of peripheral and central responses to median nerve stimulation Ricardo C. Reisin, Douglas S. Goodin, Michael J. Aminoff and Mary M. Mantle Department of Neurology, University of California, San Francisco, CA 94143 (U. S.A.) (Accepted for publication: 4 February 1988) Summary We recorded the responses to paired stimuli delivered to the median nerve at the wrist in 8 healthy adult volunteers, in order to characterize the recovery of function after a single conditioning stimulus. Responses were recorded over the nerve at the ipsilateral elbow and in the Erb's point region, over the second cervical spinous process, and over the contralateral 'hand area' of the scalp. The data from 1 subject were discarded because of possible artifactual contamination. In the others, the peripheral responses recovered both in latency and amplitude over a time period that accorded with previously published studies. We found, however, that the recovery periods for latency and amplitude of the responses recorded over the spine and scalp were prolonged compared with the corresponding values for the peripheral responses. Except for the responses recorded over the scalp, the recovery of amplitude either preceded or occurred at the same time as latency. By contrast, for the responses recorded over the scalp, there was a delay in the recovery of amplitude compared with latency. The differences in recovery period that we found at different levels of the nervous system are presumably related to structural and electrophysiological differences in afferent pathways, the presence of interposed synapses, and the intrinsic refractory properties of central neuronal populations. Key words: SEP; Peripheral recovery; Central recovery The refractory period of peripheral nerve has been investi- gated in animals and, less extensively, in humans, and its physiological basis is well known (Kimura 1981). Clinical studies have shown that the refractory period is a very sensitive measure for the early detection of peripheral nerve disease (Tackmann et al. 1974; Betts et al. 1978; Hopf and Eysholdt 1978). A comparable measure of the central recovery period may also have clinical relevance, as suggested by the dif- ferences reported in the recovery cycle between normal sub- jects and patients with multiple sclerosis (Namerow 1970). However, in only a few studies have attempts been made to define in detail the recovery cycle of the somatosensory evoked cortical responses (SEPs) to peripheral nerve stimulation, with conflicting results (Allison 1962; Shagass and Schwartz 1964; Namerow 1970). We therefore re-examined the recovery period of peripheral nerve and of the cervical and scalp-recorded SEP, and related our findings to recovery of the ability to dis- criminate paired stimuli. Method Eight healthy adult volunteers (3 men, 5 women, mean age 34 years) participated in the study. In each subject, the left Correspondence to: Dr. D.S. Goodin, Box 0114, M-794, Department of Neurology, University of California, San Francisco, CA 94143 (U.S.A.). median nerve was electrically stimulated at the wrist, and responses recorded over the elbow, Erb's point, second cervical spinous process, and contralateral scalp at C4', using the contralateral Erb's point as a reference point. Single stimuli, 0.2 msec in duration, were delivered at 5 Hz and at the maximal intensity tolerated. Mean stimulus intensity for the group was 2.6 times sensory threshold. The responses to 1000 single stimuli were recorded with an analysis time of 50 msec and averaged using a Nicolet Pathfinder II system (bandpass 30-3000 Hz). After baseline SEP responses were obtained in this way, median nerve stimulation was repeated, but with a condition- ing stimulus that preceded the test stimulus. Both conditioning and test stimuli were identical to the baseline stimulus. Re- sponses were obtained at different interstimuhis intervals (ISis). The ISI was progressively increased from 1 msec to 20 msec, 2 trials of 1000 responses being performed for each ISI. There was some individual variation in the ISis used in different subjects, and in the analysis of our results some trials had to be excluded if there was evidence of artifactual change in the baseline. At the conclusion of the experiment, a second base- line response was recorded to test stimuli delivered without preceding conditioning stimuli. The 2 baseline responses were compared, to ensure that there had been no significant change due to stimulating or recording electrode movement during the experiment, and the average of the 2 baseline responses was then used in determining the recovery function of the re- sponses. In all but 1 subject, the 2 baseline responses were 0013-4649/88/$03.50 © 1988 Elsevier Scientific Publishers Ireland, Ltd.