• Somatosensory evoked potential;
  • Pain;
  • Interstimulus interval;
  • Conduction velocity


A negative difference potential (75–240 ms poststimulus) was computed by subtracting the sural nerve-evoked somatosensory evoked potential (SEP) elicited at the pain threshold level from SEPs elicited at noxious levels. The effects of stimulus intensity and interstimulus interval on the negative difference potential amplitude plus conduction velocity measurements and a dipole source localization analysis all suggest that the negative difference potential reflects the response of neurons in the primary somatosensory cortex to inputs that arise from the nociceptive A delta peripheral afferents. Furthermore, a comparison of these results with our earlier sural nerve-evoked SEP studies suggests that these pain-related inputs to the primary somatosensory cortex are largely inhibitory.