Short latency somatosensory evoked potentials to peroneal nerve stimulation: scalp topography and the effect of different frequency filters.

Short latency SEPs to peroneal nerve stimulation were recorded from the scalp of 22 normal adults. The scalp topography and the effect of different frequency filters on these potentials were investigated. Using a wider bandpass (5-3000 Hz), this response usually consisted of 3 positive potentials (peak latencies 17, 22 and 27 msec) followed by a negative potential (peak latency 34 msec). Using a narrower bandpass (150-3000 Hz), these potentials were fractionated into subcomponents and up to 6 positive potentials were followed by an often bilobed negative potential occurring 4-10 msec earlier than the first negative potential recorded with the wider bandpass filters. The negative potential and the preceding major positive potentials were well defined and stable within and across normal subjects which suggests they will be useful in the clinical evaluation of patients with spinal cord pathology and in monitoring patients during surgery. Certain of these potentials recorded using the wider bandpass were often characterized by progressive differences in their peak latencies over the scalp. Evidence is provided which suggests that this occurred because subcomponents of these potentials, observed in recordings using the narrower bandpass had different scalp distributions. Evoked potentials were also recorded from surface electrodes placed over the spine of some of these subjects. These recordings when combined with the scalp recordings provided information concerning the conduction characteristics of SEPs from cauda equina to cerebral cortex.