Are frontal and parietal somatosensory evoked potentials functionally dissociated by changing stimulus rate?

The cortical somatosensory evoked potentials are known to be sensitive to relatively small changes in the stimulus repetition rate of the afferent nerve. However, conflicting reports exist as to whether frontally and parietally recorded potentials at a given latency show differential behaviors as a function of stimulus rate. Because such dissociations of frontal and parietal potentials can have significant implications for the SEP generation mechanisms, the present study was undertaken to further describe in detail these effects on frontal, central and parietal waveforms after median nerve stimulation. Increasing stimulus repetition rate from 1 Hz to 5 Hz had the following effects: (i) in 9 of 16 subjects, the frontal P20 diminished while parietal N20 clearly remained unaltered, (ii) the central P22 was reduced in all subjects, (iii) frontal N30 and parietal P27 were attenuated in all subjects, the average magnitude of the reductions being nearly equal for these deflections. The results support the view that changing stimulus rate can functionally dissociate frontal and parietal activity around 20 ms, indicating that several partially independent neural populations can contribute to the frontal P20. The results did not lend support for functional dissociation of frontal N30 from parietal P27.