Central scalp projection of the N30SEP source activity after median nerve stimulation

Conflicting results have been reported about abnormalities of the N30 somat osensory evoked potential (SEP) in movement disorders. In these studies, th e N30 amplitude was measured in the frontal scalp region. Our aim was to id entify the scalp electrodes recording the genuine activity of the N30 gener ator. In 18 subjects, we recorded the scalp SEPs from 19 electrodes and fou nd a negative potential around 30 ms reaching its maximal amplitude in the frontal region. However, neither simple visual inspection of the frontal tr aces nor topographic analysis could distinguish the N24 from the N30 compon ent of the frontal negativity. Brain electrical source analysis of SEPs sho wed that a four dipolar source model could well explain the scalp SEP distr ibution. We calculated the scalp field distributions of the source activiti es 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 con clude that the negative response recorded around 30 ms in the central trace s represents "genuine" N30 source activity, whereas the frontal negativity, which is higher in amplitude, is a mixture of the activities of both the N 30 and N24 sources. (C) 2000 John Wiley & Sons, Inc. Muscle Nerve 23: 353-3 60, 2000.