Effects of stimulus intensity on frontal, central and parietal somatosensory evoked potentials after median nerve...

Effects of the intensity of electrical median nerve stimulation were previously reported for the subcortical and first cortical somatosensory evoked potentials (SEPs) but not for later cortical waves whose applications in neurology have gained growing interest in recent years. This paper therefore describes the stimulus intensity effects on frontal, central and parietal SEP waveforms up to 90 msec after stimulus. The intensities were 1.5 and 2 times sensory threshold (ST), motor threshold (MT), and 1.5 and 2 times MT. Between 1.5 x ST and MT all SEP components grew in amplitude, except N60 which was essentially saturated already at 1.5 x ST. The growth was most marked for P14 and N20 whereas later potentials changed less, i.e. the slopes of the intensity-amplitude curves progressively flattened with increasing latency of SEP component. Between MT and 2 x MT no significant further alterations occurred in the early cortical potentials up to 30 msec. However, subtle changes occurred in the P40-N60 waveforms and subtraction of responses revealed a small centroparietal P35-N45 difference wave elicited only by high-intensity (2 x MT) stimulation. It is concluded that for practical purposes stimulation slightly above MT yields maximum cortical SEPs. The results are not generally compatible with the proposition that P40 and N60 are conveyed by higher threshold, small-diameter afferent fibers compared with N20. However, the P35-N45 difference wave at 2 x MT indicates that small-diameter afferent components may be embedded in the waveforms obtained at high intensity.