MIDDLE-LATENCY SOMATOSENSORY-EVOKED POTENTIALS AFTER STIMULATION OF THE RADIAL AND MEDIAN NERVES - COMPONENT STRUCTURE AND SCALP TOPOGRAPHY

Somatosensory evoked potentials (SEPs) after radial nerve stimulation are studied less frequently than those after median nerve stimulation. Therefore, little is known about their component structure and scalp topography. We investigated radial nerve SEPs after electrical stimula tion at the left wrist. For comparison, the median nerve was also stim ulated at the wrist. SEPs were recorded with 15 scalp electrodes (band pass 0.5-200 Hz) in 27 healthy subjects. The waveform of the radial ne rve SEP at a contralateral parietal lead was comparable to that of the median nerve SEP, consisting of P14, N20, P30, and N60. In spite of c omparable stimulus intensities, SEP amplitudes were smaller after radi al than after median nerve stimulation. Significant latency difference s were found only for N20 (earlier for median nerve) and P30 (earlier for radial nerve). The duration of the primary complex N20-P30 thus wa s significantly shorter for the radial nerve. Whereas N20 and P30 were present with either earlobe or frontal reference, N60 had a preroland ic maximum and was best recorded with a bipolar transverse derivation. In addition, another middle-latency negativity (N110) was found near the secondary somatosensory cortex, which had previously been describe d only for radial nerve stimulation. In standard SEP derivations, the N110 is riding on the ascending limb of the vertex negativity. It coul d best be recorded in low temporal leads versus a midline reference. T he scalp topographies of P30, N60, and N110 were similar for radial an d median nerve stimulation.