THE SCALP TOPOGRAPHY OF EARLY COMPONENTS OF SOMATOSENSORY EVOKED POTENTIAL (SEP) RECORDED SIMULTANEOUSLY FROM 16 RECORDING ELECTRODE LOCATIONS
The scalp somatosensory evoked potential (scalp-SEP) to median nerve stimulation was recorded simultaneously from 16 recording electrode locations in 31 normal subjects with MULTI-PURPOSE BIOPHYSICAL DATA PROCESSOR. The scalp topographies of the latencies of the P14, N18 and P25 components, and of the peak-to- peak amplitudes of the P14-N18 and N18-P25 were studied. According to the previous studies the Nu-N13 of nuchal-SEP was closely related to the P14 of the scalp-SEP, but in this study the latencies of the P14 recorded at frontal region were smaller than those of the Nu-N13 in some subjects. The Nu-P10 of nuchal-SEP was approximately 4 msec less in latency compared with the P14 of scalp-SEP. This latency difference would allow to cover the distance from upper cervical cord to the cerebral cortex. It seemed that the P14 potential of scalp-SEP was more closely related to the Nu-P10 than the Nu-N13 component. The P14 potential was widely distributed over the scalp and its peak latency was different at each recording location in the same subject. The peak latency of this potential was greatest at the somatosensory area (C3 or C4) contralateral to the stimulation and the greater was the increase in the distance from that area the greater the decrease in its peak latency. The N18 and P25 potentials were also widely distributed over the scalp and their peak latencies increased from front to back. Especially these differences were greatest across the central sulcus. In coronal recordings on the two sides of the scalp, there were no apparent differences over the anterior region of the central sulcus, but over the posterior region of the central sulcus, the peak latencies were smaller over the ipsilateral hemisphere than the contralateral one to the side of stimulation. The maxima of peak-to-peak amplitudes of the P14-N18 and N18-P25 were localized on the posterior contralateral quadrant of the scalp extending from Rolandic locations back to the occiput.
