Short latency somatosensory evoked potentials to median nerve stimulation: effect of low frequency filter.

The effect of low frequency filter bandpass restriction on median nerve short latency somatosensory evoked potentials was studied in 19 normal adult subjects. Components recorded with open bandpass (5-3000 Hz) were designated by numerical subscript (e.g. N20) and components recorded with restricted bandpass (150 or 300-3000 Hz) were designated by numerical superscript (e.g. N20). In open bandpass recordings, the response consisted of 3 positive potentials, the third of which was sometimes bilobed (P9, P11, P13, P14), followed by negative, positive, and negative potentials (N20, P23, N30). In restricted bandpass recordings the first three positive potentials were well defined and the N20 component was fractionated into 3 components (n16, N18, N20). N20 was followed by P21 and N23. Scalp distribution studies showed that N16 was widely distributed over the scalp but was slightly more prominent over primary somesthetic scalp regions contralateral to the side of stimulation which is consistent with an origin in caudal thalamocortical radiations. N18 and N20 were localized to somesthetic scalp regions contralateral to the side of stimulation suggesting an origin in terminal thalamocortical radiations or cerebral cortex. Open pass P23 and N30 and restricted pass P21 and N23 recorded over somesthetic scalp regions contralateral to the side of stimulation were often associated with components of similar latency but opposite polarity recorded over frontal scalp regions. Low amplitude fast frequency components were sometimes recorded between 25 and 40 msec in restricted bandpass recordings. These findings show that N20 arises in multiple generator sources of both far- and near-field origin and that multiple generators contribute to the SEP within 10-15 msec of the arrival of the volley in cerebral cortical elements. Since bandpass restriction selectively affects slow frequency activity which reflects primarily synaptic events rather than fast frequency activity which reflects axonal events, the use of bandpass restriction may aid in differentiating between these different types of activity.