Effect of stimulus rate on the cortical posterior tibial nerve SEPs: a topographic study

We performed topographical mapping of somatosensory evoked potentials (SEPs) in response to posterior tibial nerve stimulation delivered at 2, 5 and 7.5 Hz in 15 healthy subjects. P37 was significantly attenuated at 5 and 7.5 Hz and the N50 component attenuated only at 5 Hz, its amplitude remaining stable for further increases in stimulus frequency. Frontal N37 and P50 potentials showed no significant decrease when the stimulus repetition frequency was changed from 2 to 7.5 Hz. P60 showed an attenuation of the amplitude only at 7.5 Hz. Latency and scalp topographies of all cortical components examined remained unchanged for the 3 stimulus rates tested. The optimal stimulus rate for mapping of tibial nerve SEPs was lower than 5 Hz. The distinct recovery function of the contralateral. N37-P50 and ipsilateral P37-N50 responses suggests that these potentials arise from separate generators.