Human evoked potentials and C-fiber pain.

An evoked potential report by four authors has revived the question of the somatosensory specificity of the vertex potential of somatosensory evoked potentials, when these are derived by scalp conductance in response to subjectively painful skin stimulation. By a peripheral nerve-block maneuver combined with subjective sensoriperceptual reports, the authors show a late, slow positive wave in the somatosensory evoked potential whose appearance and disappearance are claimed to reflect the brain's response to a uniquely C-fiber input and to a combination of A delta and C-fiber input respectively. But their idiosyncratic interpretation of their data has merely revived an unresolved question: How much, if anything, of these familiar "ultralate," slow positivities can be ascribed to any form of neural encoding of nociception, as distinct from a nonspecific brain response to a behaviorally important stimulus?