The pain-related negative difference potential: a direct measure of central pain pathway activity or of interactions between the innocuous somatosensory and pain pathways?

Negative difference potential (NDP) is a sural nerve-evoked scalp potential derived by subtracting potentials elicited at the pain threshold level fro m those elicited at supra-pain threshold levels. Our recent work examined t he possibility that the NDP reflects a pain-related inhibition of neurons i n the innocuous somatosensory pathways. Although failing to find any eviden ce for this inhibition, these studies do present the possibility that the N DP reflects an attention- and/or task-related decrease in the innocuous som atosensory activity that is elicited by the noxious electrical stimulus. To test this hypothesis, 35 healthy subjects were given three attention/task relevance conditions presented in counterbalanced order: rate the subjectiv e magnitude of the painful aspects of the noxious electrical stimulus; rate the subjective magnitude of the non-painful aspects of the noxious electri cal stimulus; and, ignore the stimulus. Neither changes in attention nor th e task relevance of the non-painful aspects of the stimulus had any effect on NDP amplitude. These data demonstrate that the NDP does not reflect an a ttention- or task-related modulation of innocuous somatosensory activity. R ather, our evidence to date suggests that the NDP is generated by activity in the central pain pathways. (C) 1999 Editions scientifiques et medicales Elsevier SAS.