The effect of stimulus frequency on spinal and scalp somatosensory evoked potentials to stimulation of nerves in the lower limb.

The effect of variation in the stimulus frequency on spinal and cortical somatosensory evoked potentials (SSEPs) to common peroneal nerve stimulation at the knee (CPN-K) and tibial nerve stimulation at the ankle (TN-A) was studied in 11 healthy subjects. Six stimulus frequencies, 0.7, 1.5, 3.0, 5.0, 7.0 and 10.0 Hz, were used in random order. With increasing stimulus rate spinal responses remained unchanged. By contrast, early cortical responses became significantly reduced in amplitude or undetectable for stimulation frequencies above 3.0 Hz for the CPN-K and 5.0 Hz for the TN-A. In 2 subjects the configuration and the latency of CPN-K SSEPs were affected by stimulus frequency. Similar changes were not observed in TN-A SSEPs.