NEGATIVE DIFFERENCE POTENTIAL ISOLATES SCALP POTENTIALS GENERATED BY ACTIVITY IN SUPRASPINAL NOCICEPTIVE PATHWAYS

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 noxiou s levels. The effects of stimulus intensity and interstimulus interval on the negative difference potential amplitude plus conduction veloci ty measurements and a dipole source localization analysis all suggest that the negative difference potential reflects the response of neuron s in the primary somatosensory cortex to inputs that arise from the no ciceptive A delta peripheral afferents. Furthermore, a comparison of t hese results with our earlier sural nerve-evoked SEP studies suggests that these pain-related inputs to the primary somatosensory cortex are largely inhibitory.