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 from those elicited at supra-pain threshold levels. Our recent work examined the possibility that the NDP reflects a pain-related inhibition of neurons in the innocuous somatosensory pathways. Although failing to find any evidence for this inhibition, these studies do present the possibility that the NDP reflects an attention- and/or task-related decrease in the innocuous somatosensory 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 subjective magnitude of the painful aspects of the noxious electrical stimulus; rate the subjective magnitude of the non-painful aspects of the noxious electrical stimulus; and, ignore the stimulus. Neither changes in attention nor the 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 attention- or task-related modulation of innocuous somatosensory activity. Rather, 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.