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.