Central conduction in somatosensory evoked potentials: comparison of ulnar and median data and evaluation of onset versus peak methods.

To compare central conduction in ulnar and median nerve somatosensory evoked potentials (SEPs), we recorded SEPs from the neck and scalp elicited by median and ulnar nerve stimulation in 46 normal young adults. We determined the central conduction time (CCT) in each subject from peak-to-peak and onset-to-onset measurements. The mean value of the onset CCT for the ulnar nerve SEP was 6.2 +/- 0.3 msec, and for the median nerve SEP, 5.9 +/- 0.3 msec. Onset CCT was significantly longer for the ulnar nerve SEP, and there was a significant correlation between onset CCT in both median and ulnar nerve SEPs and subject height. In contrast, the mean value of the "conventional" peak CCT for the ulnar nerve SEP was 5.6 +/- 0.6 msec, and for the median nerve SEP, 5.8 +/- 0.5 msec, with no significant difference between them. In addition, the peak CCT was not correlated with subject height in the ulnar or median nerve SEPs. Our findings suggest that onset CCT measurement is superior to the conventional peak CCT measurement for ulnar as well as median nerve SEPs, and confirm that the central conduction pathway for the ulnar nerve SEP is slightly longer than that for the median nerve SEP.