Median nerve somatosensory evoked potentials (SEPs) and jerk-locked EEG back-averaging (JLA) were performed in 2 patients with left-hand myoclonus. Both SEPs were characterized by a normal parietal N20 and an enhanced N27-P27 fronto-parietal complex, but the giant central N40 described by most authors was only observed in case 1. Similarily, the aspect of JLA clearly differed in both patients. We hypothetize that both patients exhibited signs of an increased drive of the motor cortex by inputs coming from the sensory cortex. This increased drive could be mediated by the cortico-cortical connexions from areas 1-3 to area 4. In contrast, we suggest that signs of primary hyperexcitability of the motor cortex were only present in case 1, as suggested by the presence of an enhanced N40 in SEPs and the results of JLA studies.
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