Latency differences of N20, P40/N60, P100/N140 SEP components after stimulation of proximal and distal sites of the median nerve.

The latencies of SEP N20, P40, N60, P100, and N140 components were measured after stimulation of two different sites, and the differences in relation to conduction velocity and their central functions are discussed. Subjects were 8 healthy right-handed males (age 22-31 years, height 164-184 cm). An electrical pulse of 200micro sec duration with an intensity of 2 times the motor threshold was delivered to the wrist and to the elbow alternately at a random rate of 0.1 to 0.3 Hz. Recording electrodes were Cz', C3', and C4' referenced to linked ears. Analysis time was 50 msec before and 450 msec after the stimulus. The band pass was 0.5 Hz to 2 kHz. Subjects were asked to mentally count the number of stimuli. The averaging was interrupted after every 16 to 24 stimuli and checked to determine whether the subject was attentive to the stimuli by verifying the number of stimuli counted. A total of 100 responses each from elbow and wrist stimuli were averaged. Differences in peak latency between elbow and wrist stimuli for N20, P40, N60, P100, and N140 were 3.7 +/- 0.7 msec, 5.0 +/- 1.8, 4.3 +/- 1.2, 8.1 +/- 6.3, and 7.4 +/- 2.6 msec, respectively. According to the latency differences, SEP components can be divided into 3 groups: the shortest difference for N20, medium difference for P40 and N60 and longest difference for P100 and N140. Similar latency differences and similar potential distribution for P40 and N60, and for P100 and N140, and their differences from N20 confirmed that each of N20, P40/N60, and P100/N140 has a different function centrally. In addition, central processing time was longer with the more distal site stimulation.