Influence of stimulus repetition rate on cortical somatosensory potentials evoked by median nerve stimulation: implications for generation mechanisms.

Despite growing clinical and experimental interest in the cortical components of somatosensory evoked potentials (SEPs) little is known about their physiological dynamics, e.g. with changing stimulation parameters. This paper reports the influence of varying stimulus repetition rate from 0.5 to 5 Hz on cortical SEPs up to 60-msec latency after right median nerve stimulation, separately analyzed at frontal (F3), central (C3) and parietal (P3) electrodes. The amplitudes of early frontal P20 and N25, central P14 and N18, and parietal N20 did not change with stimulation rate. Later deflections were significantly modified when their amplitudes were determined with respect to the baseline: at F3 negative N30 and N60 diminished and positive P40 was enhanced with increasing rate of stimulation. At P3 the effects were the reverse, so that positive P27 and P45 were attenuated while negative N34 and N60 were enhanced. At C3 both positive P22 and P40 and negative N60 were reduced. However, the corresponding peak-to-peak amplitudes changed much less. We conclude that SEP waveforms following the earliest cortical deflections are very sensitive to small changes in stimulation frequency. The opposite changes at F3 compared with P3 probably represent the opposite scalp field poles from horizontally oriented generator(s) located within the primary sensorimotor cortex (SMI). We suggest that the rate effects are partly due to selective sensitivity of postexcitatory inhibitory postsynaptic potentials to stimulation frequency.