Recovery functions of fast frequency potentials in the initial negative wave of median SEP

Following far-field potential of P14 after median nerve stimulation, we identified several small wavelets, designated here as fast frequency potentials or FFP, over the ascending and descending phases of the major negative wave of 'N20.' In 10 normal subjects who had well defined FFP, namely N16, N17, N18 and N19, we studied recovery functions of FFP by applying paired (condition and test) stimuli of various interstimulus intervals (ISIs) ranging from 3 to 150 msec. Recovery of FFP was determined by the difference wave forms, i.e., subtracting the response of conditioning stimuli from the response to paired stimuli. Measuring latencies and amplitude of FFP and also of the far-field potential of P14, we found recoveries of P14 and each FFP to be different; P14 recovered with the shortest ISI while progressively longer ISIs were required for full recovery of the later FFP. This finding suggests the existence of progressively increased number of interspersed synapses from the early to late FFP. It is unlikely that FFP represents passage of the afferent volley from one anatomical site to the next, but rather that they are cortically generated via a closely situated polysynaptic network. The polysynaptic nature of FFP is consistent with the observation that they disappear in sleep and also during anesthesia. The study indicates that FFPs represent different physioanatomical systems from the primary negative wave of 'N20' that results from specific sensory input to the primary sensory cortex.