Event related brain potentials and human pain: a first objective overview.

Since 1960 systematic studies of the human scalp-conducted cerebral slow-wave response to painful stimulation have shown only amplitude augmentation of the vertex components of the somatosensory evoked potential (SEP) to be indicators of subjective perception of a noxious or aversive quality in the stimulus. The vertex potential (VP) of the SEP occurs relatively late after onset of either transient or maintained stimuli (100-400 ms depending on stimulus mode and site), is amplitude-focal at vertex on scalp, and may be asymmetrically distributed hemispherically for unilateral stimulation of the hands. Its functional neuroanatomy seems undeterminable by scalp macroelectrodes, and its relationship to analogous vertex potentials in the auditory and visual modalities (AEP and VEP) unknown. Studies of the nociceptive SEP (SEPn) since 1977 concur that VP amplitude is more readily correlatable with subjective pain magnitude estimate than with objective stimulus parameters. They also suggest that the VP is amplitude-sensitive to (a) interstimulus-intervals less than about 350 ms; (b) analgesics and their antagonists; and (c) subjective cognitive status with regard to both the expected aversiveness of the stimulus and the previous experience of chronic pain. These studies have included electrical stimulation of toothpulp and teeth, mechanical and transcutaneous electrical stimulation of palmar and digital glabrous skin, noxious thermal stimulation of hands and forearms, and electrical stimulation of lips, fingers, toes, and anogenital perineum of both sexes. They have been done in the context of both classical and signal-detectability (TSD) methods for concurrent reports of painful versus painless, and in the methodological contexts of conventionally signal-averaged SEP and of single-epoch SEP recovered both raw and processed. Studies designed to analyze differences in the early-intermediate (25-95 ms) and late (500-1000 ms) time-segments of SEP for painful and painless stimulation have provided no convincing evidence of cortical nociceptive signals. Therefore existing data on the SEPn imply that the only cortical slow-wave sign of nociception reflects perceptual-cognitive and endogenous, rather than sensory-discriminative and exogenous, aspects of the conscious pain experience.