Central scalp projection of the N30 SEP source activity after median nerve stimulation

Conflicting results have been reported about abnormalities of the N30 somatosensory evoked potential (SEP) in movement disorders. In these studies, the N30 amplitude was measured in the frontal scalp region. Our aim was to identify the scalp electrodes recording the genuine activity of the N30 generator. In 18 subjects, we recorded the scalp SEPs from 19 electrodes and found a negative potential around 30 ms reaching its maximal amplitude in the frontal region. However, neither simple visual inspection of the frontal traces nor topographic analysis could distinguish the N24 from the N30 component of the frontal negativity. Brain electrical source analysis of SEPs showed that a four dipolar source model could well explain the scalp SEP distribution. We calculated the scalp field distributions of the source activities as modeled from the scalp recordings and observed that the maximal field distribution reflecting the activity of the N30 source was in the central region, whereas that reflecting the N24 source activity was frontal. We conclude that the negative response recorded around 30 ms in the central traces represents "genuine" N30 source activity, whereas the frontal negativity, which is higher in amplitude, is a mixture of the activities of both the N30 and N24 sources.